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Interfacing Methods and
Circuits
Chapter 11
Introduction
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A sensor/actuator can rarely operate on its own.
Exceptions exist (bimetal sensors)
Often a circuit of some sort is involved.
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can be as simple as adding a power source or a
transformer
can involve amplification, impedance matching, signal
conditioning and other such functions.
often, a digital output is required or desirable so that
an A/D may be needed
The same considerations apply to actuators
Introduction
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The considerations of interfacing should be part of the
process of selecting a device for a particular application
since this can simplify the process considerably.
Example: if a digital device exists it would be wasteful to
select an equivalent analog device and add the required
circuitry to convert its output to a digital format.
The likely outcome is a more cumbersome, expensive
system which may take more time to produce.
Alternative sensing strategies and alternative sensors
should always be considered before settling on a
particular solution
Introduction
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Many types of sensors and actuators based on
very different principles
There are commonalities between them in terms
of interfacing requirements
Most sensors’ outputs are electric (voltage,
current, resistance)
These can be measured directly after proper
signal conditioning and, perhaps, amplification.
If the output is a capacitance or an inductance require additional circuitry such as oscillators
Introduction
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There is a large range of signal levels in
sensors.
A thermocouple’s output is of the order of
microvolts
An LVDT may easily produce 5V AC.
A piezoelectric actuator may require a few
hundred volts to operate (very little current)
A solenoid valve operates at perhaps 12-24V
with currents that may exceed a few amperes.
How does one measure these signals?
Introduction
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The circuitry required to drive and to interface
them to, say a microprocessor are vastly
different
Require special attention on the part of the
engineer.
Must consider such issues as response
(electrical and mechanical), spans and power
dissipation as well as power quality and
availability.
Example: Systems connected to the grid and
cordless systems have different requirements
and considerations in terms of operation and
safety.
Purpose
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Discuss general issues associated with
interfacing
Outline general interfacing circuits the engineer
is likely to be exposed to.
No general discussion however can prepare one
for all eventualities
It should be recognized that there are both
exceptions to the rules and extensions to the
methods discussed here.
Purpose
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Example: an A/D is a simple – if not inexpensive –
method of digitizing a signal for the purpose of
interfacing
This approach however may not be necessary and too
expensive in some cases.
Suppose the hall element senses the teeth on a gear.
The signal from the hall element is an ac voltage - only
the peaks are necessary to sense the teeth.
In this case a simple peak detector may be adequate.
An A/D converted will not provide any additional benefit
and is a much more complex and expensive solution.
On the other hand, if a microprocessor is used and an
A/D is available it may be acceptable to use it for this
purpose
Content
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Operational amplifiers and power
amplifiers
A/D and D/A conversion circuits
Bridge circuits
Data transmission
Excitation circuits
Noise and interference
Amplifiers
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An amplifier is a device that amplifies a signal –
almost always a voltage
The low voltage output of a sensor, say of a
thermocouple, may be amplified to a level
required by a controller or a display.
Amplification may be quite large – sometimes of
the order of 106 or it may be quite small or even
smaller than one, depending on the need of the
sensor.
Amplifiers
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Amplifiers can also be used for impedance matching
purposes even when no amplification is needed
May be used for the sole purpose of signal conditioning,
signal translation or for isolation
Power amplifiers, which usually connect to actuators,
serve similar purposes beyond providing the power
necessary to drive the actuator.
Amplifiers can be very simple – a transistor with its
associated biasing network or may involve many
amplification stages of varying complexity.
Amplifiers are sometimes incorporated in the sensor
Amplifiers
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We will use the operational amplifier as the basic
building block for amplification.
Operational amplifiers are basic devices and may be
viewed as components.
An engineer, especially when interfacing sensor is not
likely to dwell into the design of electronic circuits below
the level of operational amplifiers.
Although there are instances where this may be done to
great advantage, op-amps are almost always a better,
less expensive and higher performance choice.
Same idea for power amplifiers
Operational Amplifiers
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Operational amplifier is a fairly complex
electronic circuit but:
It is based on the idea of the differential voltage
amplifier shown in Figure 11.1.
Based on simple transistors,
The output is a function of the difference
between the two inputs.
Assuming the output to be zero when both
inputs are at zero potential, the operation is as
follows:
Differential amplifier
Operational amplifier
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When the voltage on the base of Q1 increases,
its bias increases while that on Q2 decreases
because of the common emitter resistance.
Q1 conducts more than Q2 and the output is
positive with respect to ground.
If the sequence is inverted, the opposite occurs.
If, both inputs increase or decrease equally,
there will be no change in output.
Operational amplifier
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An operational amplifier is much more complex
than this but operates on the same principle.
It contains additional circuitry (such as
temperature and drift compensation, output
amplifiers, etc.)
These are of no interest to us other than the fact
that they affect the specifications of the op-amp.
There are also various modifications to op-amps
that allow them to operate under certain
conditions or to perform specific functions.
Operational amplifier
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Some are “low noise” devices
Others can operate from a single polarity source.
If the input transistors are replaced with FETs,
the input impedance increases considerably
requiring even lower input currents from sensors
These are important but are variations of the
basic circuit.
We will consider it as a simple block shown in
Figure 11.2 and discuss its general properties
based on this diagram
The operational amplifier
Op-Amps - properties
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Differential voltage gain: the amplification of
the op-amp of the difference between the two
inputs:
Also called the open loop gain
in a good amplifier it should be as high as
possible.
Gains of 106 or higher are common.
An ideal amplifier is said to have infinite gain.
Op-Amps - properties
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Common-mode voltage gain.
By virtue of the differential nature of the
amplifier, this gain should be zero.
Practical amplifiers may have a small common
mode gain because of the mismatch between
the two channels but this should be small.
Common mode voltage gain is indicated as Acm.
The concept is shown in Figure 11.3.
Common mode signal an output
Op-amps - properties
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More common to specify the term Common
Mode Rejection Ratio (CMRR)
CMRR is the ratio between Ad and Acm:
CMRR =
Ad
Acm
In an ideal amplifier this is infinite.
A good amplifier will have a CMRR that is very high
Op-amps - properties
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Bandwidth: the range of frequencies that can
be amplified.
Usually the amplifier operates down to dc and
has a flat response up to a maximum
frequency at which output power is down by
3dB.
An ideal amplifier will have an infinite
bandwidth.
The open gain bandwidth of a practical
amplifier is fairly low
A more important quantity is the bandwidth at
the actual gain.
Op-amps - properties
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This may be seen in Figure 11.4
The lower the gain, the higher the bandwidth.
Data sheets therefore cite what is called the
gain-bandwidth product.
This indicates the frequency at which the gain
drops to one and is also called the unity gain
frequency.
In Figure 11.4:
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BW (open loop) is 2.5 kHz
Unity Gain Frequency is 5 MHz
Bandwidth of op-amp
Op-amps - properties
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Slew Rate: the rate of change of the output in
response to a change in input, given in V/s.
If a signal at the input changes faster than the
slew rate, the output will lag behind it and a
distorted signal will be obtained.
This limits the usable frequency range of the
amplifier.
For example, an ideal square wave will have a
rising and dropping slope at the output defined
by the slew rate.
Op-amps - properties
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Input impedance: the impedance seen by the
sensor when connected to the op-amp.
Typically this impedance is high (ideally infinite)
It varies with frequency.
Typical impedances for conventional amplifiers
is at least 1 M but it can be of the order of
hundreds of M for FET input amplifiers.
This impedance defines the current needed to
drive the amplifier and hence the load it
represents to the sensor.
Op-amps - properties
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Output impedance: the impedance seen by the
load.
Ideally this should be zero since then the output
voltage of the amplifier does not vary with the
load
In practice it is finite and depends on gain.
Usually, output impedance is given for open loop
whereas at lower gains the impedance is lower.
A good amplifier will have an output resistance
lower than 1.
Op-amps - properties
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Temperature and noise refer to variations of
output with temperature and noise
characteristics of the device respectively.
These are provided by the data sheet for the opamp and are usually very small.
For low signals, noise can be important while
temperature drift, if unacceptable must be
compensated for through external circuits.
Op-amps - properties
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Power requirements. The classical op-amp is
built so that its output can swing between ±Vcc
Dual supply operation is common in op-amps
The limits can be as low as ±3V (or lower) and
as high as ±35V (sometimes higher).
Many op-amps are designed for single supply
operation of less than 3V and some can be used
in single supply or dual supply modes.
Op-amps - properties
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Current through the amplifier is an important
consideration, especially the quiescent current (no load)
Gives a good indication of power needed to drive it.
Particularly important in battery operated devices. The
current under load will depend on the application but it is
usually fairly small – a few mA.
In selection of a power supply for op-amps, care should
be taken with the noise that the power supply can inject
into the amplifier.
The effect of the power supply on the amplifier is
specified through the power supply rejection ratio
(PSRR) of the specific amplifier.
Op-amps - data sheets
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The 741 op-amp is an older, general
purpose amplifier.
It is a fairly low performance device but is
characteristic of the low-end amplifiers.
Very common and quite suitable for many
applications. LM741.PDF
Op-amps - data sheets
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The TLC27L2C is a dual, low power opamp, suited for battery operated devices
Part of a series of amplifiers using FETs as
input transistors TLC27L2C
Inverting and noninverting
amplifiers
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Performance of the amplifier depends on how it
is used and, in particular on the gain of the
amplifier.
In practical circuits, the open loop gain is not
useful and a specific gain must be established.
For example, we might have a 50mV output
(maximum) from a sensor and require this
output to be amplified, say by 100 to obtain 5V
(maximum) for connection to an A/D.
This can be done with one of the two basic
circuits shown in Figure 11.5, establish a means
of negative feedback to reduce the gain
Inverting op-amp
Non-inverting op-amp
Inverting op-amp
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The output is inverted with respect to the input
(180 out of phase).
The feedback resistor, Rf, feeds back some of
this output to the input, effectively reducing the
gain.
The gain of the amplifier is now given as:
Av = 
Rf
RI
In the case shown here this is exactly –10
Inverting op-amp
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The input impedance of the amplifier is given as
Ri = RI
Here it is equal to 1 k.
If a higher resistance is needed, larger resistances might be
needed
Or, perhaps, a different amplifier will be needed
(noninverting amplifier)
Inverting op-amp
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The output impedance of the amplifier is
given as
RI + Rf Open loop output impedance
Ro =
RIAOL
AOL is the open loop gain as listed on the data sheet
Open loop gain is the open loop gain at the frequency at
which the device is operated
Inverting op-amp
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Example, for the LM741 amplifier, the open loop
output impedance is 75 and the open loop gain
at 1 kHz is 1000. This gives an output
impedance of:
1000 + 10000 75
Ro =
= 0.825 
10001000
The bandwidth is also influenced by the feedback:
unity gain frequencyRI
BW =
RI + Rf
Non-inverting amplifier
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The non-inverting amplifier gain is:
Rf
Av = 1 +
RI
For the circuit shown, this is 11
The gain is slightly larger than for the noninverting
amplifier for the same values of R.
The main difference however is in input impedance.
Non-inverting amplifier
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Input impedance is:
Ri = Rop Aol
RI
RI + Rf
Rop is the input impedance of the op-amp as given in the
spec sheet
Aol is the open loop gain of the amplifier.
Assuming an open loop impedance of 1 M (modest
value) and an open loop gain of 106, we get an input
impedance of 1011 . (almost ideal)
Non-inverting amplifier
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The output impedance and bandwidth are the
same as for the inverting amplifier.
The main reason to use a noninverting amplifier
is that its input impedance is very large making it
almost ideal for many sensors.
There are other properties that need to be
considered for proper design such as output
current and load resistance but these will be
omitted here for the sake of brevity.
The voltage follower
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The feedback resistor in the noninverting
amplifier is set to zero
The circuit in Figure 11.6 is obtained.
The gain is one.
This circuit does not amplify.
Why use it?
The voltage follower
Voltage follower
The input impedance now is very large and equal
to:
Ri = Rop Aol
The output impedance is very small and equal to:
Open loop output impedance
Ro =
AOL
Voltage follower
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The value of the voltage follower is to
serve in impedance matching.
One can use this circuit to connect, say, a
capacitive sensor or, an electret
microphone.
If amplification is necessary, the voltage
follower may be followed by an inverting or
noninverting amplifier
Instrumentation amplifier
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The instrumentation amplifier is a modified opamp
Its gain is finite and both inputs are available to
signals.
These amplifiers are available as single devices
To understand how they operate, one should
view them as being made of three op-amps (it is
possible to make them with two op-amps or
even with a single op-amp), as shown in Figure
11.7.
Instrumentation amplifier
Instrumentation amplifier
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The gain of an amplifier of this type is:
R
Av = 1 + 2R 3
Ra R2
In a commercial instrumentation amplifier all
resistances are internal and produce a gain usually
around 100.
Ra is external and can be set by the user to obtain the
gain required.
Instrumentation amplifier
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The output of the instrumentation amplifier is
Vo = Av V+  V
The main use of this amplifier is to obtain an output
proportional to difference between inputs.
Important in differential sensors, especially when
one sensor is used to sense the stimulus and an
identical sensor is used for reference (such as when
temperature compensation is needed)
Instrumentation amplifier
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Each of the inputs has the high impedance of
the amplifier used
The output impedance is low (inverting amp.)
The main problem in a circuit of this type is that
the CMRR depends on the matching of the
resistances (R, R2 and R3) in each section of the
circuit.
These are internal and are adjusted during
production to obtain the required CMRR.
Charge amplifier
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The so-called charge amplifier is shown in
Figure 11.8.
Charge cannot be amplified but the output
voltage can be made proportional to charge as
follows:
The output of the inverting amplifier is:
Av = 
1/jC
Rf
C
=
= 0
RI
1/jC0
C
C0 is the capacitance connected across the inverting input.
Charge amplifier
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Assuming that a change in charge occurs on the
capacitor, equal to Q = C0V, the output
voltage may be written as
Q
C
0
Vo =  V = 
C
C
In effect the charge generated at the input is amplified
Charge amplifier
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If C is small, a small change in charge at the
input can generate a large voltage swing in the
output.
The main method of connecting capacitive
sensors such as pyroelectric sensors whose
output is low (piezoelectric sensors, on the other
hand produce a higher voltage).
It is necessary for the input impedance to be
very high and care must be taken in connections
(such as the use of very good capacitors).
Commercial charge amplifiers use FET
transistors to ensure the necessary high input
impedance.
Charge amplifier
Current amplifier
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Another example of the use of an amplifier
to a specific end is the current amplifier
Current amplifier
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The input voltage is Vi=ir.
Just like the inverting amplifier, the output
now is
Vo =  Vi R =  iR
r
Useful with very low impedance sensors.
May be used with thermocouples whose impedance
can be trivially low.
They may be connected directly (r then represents
the resistance of the thermocouple).
The output is a direct function of the current the
thermocouple produces which can be fairly large
The comparator
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An op-amp operated in open loop mode
Because its gain is so high, a very small signal
at the input will saturate the output.
For practically any input, the output will be either
+Vcc or –Vcc. Consider Figure 11.10.
The negative input is set at a voltage V and
V+=0. Therefore the output is AolV=Vcc.
Suppose we increase V+. Output is (V+V)Aol.
As long as V+<V-, the output remains –Vcc. If
V+>V, the output changes to + Vcc.
The comparator
The comparator
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The function of this device is to compare
the two inputs and to indicate which one is
higher.
The comparator is useful beyond simple
comparison.
It will be used extensively in A/D and D/A
conversion of signals and in many other
aspects of sensing and actuation
Power amplifiers
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A power amplifier is a device or circuit whose
power output is the input power multiplied by a
power gain:
Po = Pi Ap
That is, the amplifier is capable of boosting the
power level of a signal to match the needs of an
actuator.
Power amplifiers
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The obvious use of power amplifiers is in driving
actuators, (speakers, voice coil actuators and
solenoid actuators and motors).
The power amplifier is really either a voltage
amplifier or a current amplifier (also called
transconductance amplifier).
In a voltage amplifier, the input signal is a
voltage.
This voltage is amplifier and in the final stage a
sufficiently high current provided so that the
required power is met.
Power amplifiers
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In a current amplifier the opposite occurs.
Power amplifiers are divided into linear and
PWM (pulse width modulated) amplifiers.
In a linear amplifier, the output (voltage) is a
linear function of the input and can be anything
between ±Vcc.
In a PWM amplifier the output is either Vcc or
zero and the power delivered is set by the time
the output is on. The latter is controlled by the
width of the pulse that controls the output.
Linear power amplifiers
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First step is to amplify the signal to the required
output.
Can be done using any amplifier
We shall assume an op-amp was used for this
purpose.
Then this voltage is applied to an “output stage”
It does not need to amplify but, rather, supplies
the necessary current.
A simple example is shown in Figure 11.11.
Linear power amplifier
Class A linear amplifier
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This is the so called Class A power amplifier.
Set for a gain of 101 (noninverting amplifier).
The output then drives the transistor whose
output will swing, at most between 0 and V
Will supply a current which is V/RL
Class A designation indicates amplifiers for
which the output stage is always conducting as
in the case above. Also assumes output does
not saturate.
The BJT can be replaced with a MOSFET for
higher currents.
Class A linear amplifier
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This type of amplifier is sometimes used to drive
relatively small loads such as light indicators,
small dc motors and some solenoid valves.
In some cases the amplification is set high
enough to saturate the amplifier in which case
the amplifier operates as an on/off circuit rather
than a class A amplifier
Typically used to turn on/off relays, lights,
motors, etc.
Class B amplifier
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A Class B or push-pull amplifier is shown in
Figure 11.12.
It is usually a better choice.
It operates exactly as in the previous case
except that under no input, the output is zero
and there is no conduction in the transistors (or
MOSFETs).
When the input is positive, the upper transistor
conducts supplying the load and when the input
is negative, the lower transistor supplies the
load.
Class-B (push-pull) power
amplifier
Class B amplifier
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The voltage in the load can swing between +Vcc
and –Vcc
The current is again defined by the load.
The output stage is made of a pair of power
transistors, one PNP and one NPN (or of a P
and an N type MOSFET).
There are many variations of the basic
amplifiers.
For example, feedback may be added and it is
common to protect the output stage from short
circuits as well as from spikes due to inductive
and capacitive loads.
Class B amplifier
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In terms of performance, the obvious are the power
output and the type and level of input.
For example, an amplifier may be specified as supplying
100W for a 1V input.
Next is the distortion level.
Distortions are specified as a percentage of output.
The most common specification is the THD (total
harmonic distortions) as % of output.
A good amplifier will have less than, say, 0.1% THD.
Other specifications are temperature rise and output
impedance of the amplifier (must match load).
Class B amplifier
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Power amplifiers of various power level exist
either as integrated circuits or as discrete
components circuits.
Usually the discrete circuits can supply higher
powers.
An example of an integrated amplifier is the
TDA2040 which can supply 20W and is
designed for use as an audio amplifier.
Nevertheless it can drive other loads such as
light bulbs, small motors, etc.
PWM amplifiers
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The PWM approach is shown schematically in
Figure 11.13.
The power transistors are driven on and off so
that the voltage on the load can only be zero or
Vcc.
The time the power is on is controlled by the
timing circuit.
This defines the average power at the load.
The PWM principle
PWM amplifiers
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The pulse width modulator is an oscillator which
generates a square wave whose duty cycle can
be controlled based on the required power.
For example, in Figure 11.14, the timing circuit
defines for how long the input signal is
connected to the transistor, hence for how long it
conducts.
The power in the load is a function of this timing.
This circuit is not particularly useful but others
are.
PWM driving of a load
PWM driving
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Figure 11.15 shows an example often used to
control speed and direction of small dc motors.
It is called an H-bridge for obvious reasons.
A pulse of constant amplitude but varying duty
cycle connected to point A, will drive MOSFETs
1 and 4, turning the motor into one direction.
The duty cycle defines the average current in the
motor and hence its speed.
Connecting to point B, turns on MOSFETs 2 and
3 reversing the process.
H-Bridge driven from a PWM
source
H-bridge PWM driver
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Some precautions must be taken to ensure that
only opposite transistors conduct
This is one of the most common circuits used for
bidirectional control of motors and other
actuators.
The controllers for these devices can be a small
microprocessor
Integrated PWM circuits and controllers are
available commercially
A/D and D/A converters
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These are the means by which a signal can be
converted from analog to digital or from digital to
analog as necessary.
The idea is obvious but implementation can be
complex.
There are certain types of D/A and A/D that are
trivially simple.
We will start with these and only then discuss
some of the more complex schemes.
In certain cases one of these simple methods is
sufficient.
A/D and D/A converters
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Analog to digital and, to a lesser extent, digital to analog
conversion are common in sensing systems since most
sensors and actuators are analog devices and most
controllers are digital.
Most A/Ds required voltages much above the output of
some sensors.
Often the output from the sensor must be amplified first
and only then converted.
This leads to errors and noise and has resulted in the
development of direct digitization methods based on
oscillators (to be discussed below).
Threshold digitization
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In some cases, an analog signal represents
simple data such as the presence of something.
For example, in chapter 5 we discussed the
detection of teeth on a gear using a hall
element.
The signal obtained is quite small and looks
more or less sinusoidal with the peaks
representing the presence of the teeth.
In such a case it is sufficient to use a threshold
amplifier which will then produce a digital output.
An example is shown in Figure 11.16a.
Threshold digitization
Threshold digitization
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
The output from the the hall element varies from
100mV to 150mV. This signal can be fed into a
comparator as shown in Figure 11.17
The negative input is set by the resistors to
0.13V.
Normally the output is zero until the voltage on
the positive input rises above the threshold.
When the input dips below 0.13V the output
goes back to zero.
The output in Figure 11.16b is obtained and
now, each pulse represents a tooth on the gear.
Comparator threshold
digitization
Threshold digitization




Counting the teeth in a given time can give the
speed of rotation of the gear or other data.
A missing tooth, the corresponding pulse will be
represented by a missing pulse
This method is very effective when voltages at
the input change across the comparison point
At the comparison point itself, the output of the
comparator is not properly defined and the
output can change states back and forth creating
pulses which are spurious.
Threshold digitization




To avoid this a hysteresis is added to the
comparator so that the transition from low to
high occurs say, at V0 and the transition from
high to low occurs at V0-V.
Hysteresis can be added to comparators
through external components.
Another approach is to use of a Schmitt trigger.
The Schmitt trigger is essentially a digital
comparator with a built in hysteresis as
described above whose transition is around
Vcc/2.
Threshold digitization




Threshold digitization is a very simple method of
digitization and is sufficient for many
applications.
It is commonly used for the purpose above but
also in flow meters in which a rotating paddle
operates a hall element or another magnetic
sensor
It is also useful for optical sensors which use the
idea of interruption of the beam.
It is not however suitable for measuring the level
of a signal such as voltage from a thermocouple.
Direct voltage to frequency
conversion



In many sensors, the output is too small to use
the method above or to be sent over normal
lines for any distance.
In such cases a voltage to frequency conversion
can be performed at the location of the sensor
and the digital signal then transferred over the
line to the controller.
The output now is not voltage but rather a
frequency which is directly proportional to
voltage (or current).
Direct voltage to frequency
conversion




These voltage-to-frequency converters or voltage
controlled oscillators are relatively simple and accurate
circuits and have been used for other purposes.
Their main advantage over the threshold method above
is that lower levels of signals may be involved and the
problems with noisy transitions around the comparison
voltage are eliminated.
A circuit of this type is shown in Figure 11.18, as used
with a light sensor.
The circuit is an op-amp integrator.
Direct voltage to frequency
conversion
Direct voltage to frequency
conversion






The voltage across the capacitor is the integral of the
current in the noninverting leg of the amplifier.
This current is proportional to the voltage across R2.
As the voltage on the capacitor rises, a threshold circuit
checks this voltage
When the threshold has been reached, an electronic
switch shorts the capacitor and discharges it.
The switch then opens and allows the capacitor to
recharge.
The voltage on the capacitor is a triangular shape whose
width (i.e. the integration time) depends on the voltage at
the noninverting input.
Direct voltage to frequency
conversion






If no light is present on the sensor, it has a dark
resistance and the voltage at the noninverting input will
have a certain value.
The output of the amplifier changes at a frequency f1.
If now light falls on the sensor, its resistance goes down
and the total resistance at the noninverting input falls.
This reduces the input voltage and hence the integration
time until the capacitor reaches the threshold increases.
The result is that the amplifier changes state slower and
the output is a lower frequency f2.
Since small changes in frequency can be easily
detected, this is a very sensitive method of digitization
for small signal sensors.
Voltage to frequency conversion



Other V/F converters require a much higher
voltage and they are more suitable for A/D
conversion after amplification of the lower
signals or for sensors whose output is high to
begin with. There are two basic methods.
One type is essentially a free running oscillator
whose frequency can be controlled by the input
voltage.
The second is a modification of Figure 11.18
and is called a charge-balance V/F converter.
Voltage to frequency conversion






A simple V/F method is shown in Figure 11.19.
It consists of a square wave oscillator (called a
multivibrator) and a control circuit.
The multivibrator operates by charging and discharging a
capacitor.
The on/off times of the waveform (hence frequency) are
controlled by charging/discharging times of the capacitor.
To control frequency voltage to be converted is amplified
and fed as currents to the bases of the two transistors.
The larger the base current, the larger the collector
current and the faster the charge/discharge and hence
the higher the frequency of the multivibrator.
Simple voltage to frequency
conversion - multivibrator
V/F conversion





A different approach is shown in Figure 11.20.
The amplifier acts as an integrator and the FET across
the capacitor is the switch.
The capacitor charges at a rate proportional to the
current I=V0/R which is proportional to the voltage to be
converted.
When the output has reached the threshold voltage of
the schmitt trigger it changes state, turning on and this
turns on the FET switch.
Discharging of the capacitor occurs and the output
resets to restart the process. Again, as before, only
relatively large level voltages can be converted.
Simple voltage to frequency
conversion - integrator
Dual slope A/D converter


The simpler (and slower) of the true A/D
converters
Based on the following principle: a
capacitor is charged from the voltage to be
converted through a resistor, for a fixed,
predetermined time T. The capacitor
reaches a voltage VT which is:
VT = Vin T
RC
Dual slope A/D converter



At time T, Vin is disconnected
A negative reference voltage of known
magnitude is connected to the capacitor through
the same resistor.
This discharges the capacitor down to zero in a
time T
 VT = Vref T
RC
Dual slope A/D converter

Since these are equal in magnitude we have:
Vin T = Vref T
RC
RC

Vin T
=
Vref
T
In addition, a fixed frequency clock is turned on at the
beginning of the discharge cycle and off at the end of the
discharge cycle. Since T and T are known and the counter
knows exactly how many pulses have been counted, this
count is the digital representation of the input voltage.
A schematic diagram of a dual slope converter based on these
principles is shown in Figure 11.21.
Dual slope A/D conversion
Dual slope A/D converter



The method is rather slow with approximately
1/2T conversions per second.
It is also limited in accuracy by the timing
measurements, accuracy of the analog devices
and, of course, by noise.
High frequency noise is reduced by the
integration process and low frequency noise is
proportional to T (the smaller T the less low
frequency noise).
Dual slope A/D converter




The dual slope A/D is the method of choice for many
sensing applications in spite of its rather slow response
because it is simple and readily built from standard
components.
For most sensors its performance and noise
characteristics is quite sufficient
Because of the integration involved, it tends to smooth
variations in the signal during the integration.
The method is also used in digital voltmeters and other
digital instruments.
Successive approximation
A/D




This is the method of choice in A/D converter
components and in many microprocessors.
It is available in many off the shelf components
with varying degrees of accuracy
Depending on the number of bits of resolution it
may resolve down to a few microvolt.
The basic structure is shown in Figure 11.22. It
consists of a precision comparator, a shift
register a digital to analog converter and a
precision reference voltage Vref.
Successive approximation A/D
conversion
Successive approximation A/D





The operation is as follows:
First, all registers are cleared, which forces the
comparator to HIGH.
This forces a 1 into the MSB of the register.
The D/A generates an analog voltage Va which
for MSB=1 is half the full scale input.
This is compared to Vin. If Vin is larger than Va,
the output stays high and the clock shifts this
into the next bit into the register.
Successive approximation A/D






The register now shows 1100000000.
If it is smaller than Vin, the output goes low and the
register shows 010000000.
Assuming that the input is still higher, the D/A generates
a voltage Va=(1/2+1/4)Vfs.
If this is higher than the input, the register will show
011000000 but if it is lower, it will show 11100000 and so
on, until, after n steps the final result will be obtained.
The data is read from the shift register and represents
the voltage digitally.
This digital value can now be shifted out and used by the
controller.
Successive approximation A/D





A/D of this type exists with resolution of up to 14
bits with 8 and 10 bits being quite common.
An 8 bit A/D has a resolution of: Vin/28=0.004Vin.
For a 5V full scale, the resolution is 20 mV.
This may not be sufficient for low level signals in
which case a 10, 12 or 12 bit A/D may be used
(a 14 bit A/D has a resolution of 0.3mV).
There are also techniques of extending this
resolution but it is almost always necessary to
amplify signals from devices such as
thermocouples if they must be digitized.
Successive approximation A/D






The advantage of the successive approximation A/D is
that the conversion is done in n steps (fixed)
It is much faster than other methods.
On the other hand the accuracy of the device depends
heavily on the comparator and the D/A converter.
Commercial devices are fairly expensive, especially if
more than 10 bits are needed.
This type of A/D has been incorporated directly into
microprocessors and can sometimes be used for
sensing as part of the overall circuitry.
Some microprocessors have multiple A/D channels.
Digital to Analog Conversion




Digital to analog conversion is less often used
with sensors but is sometimes used with
actuators.
This occurs when a digital device, such as a
microprocessor must provide an analog output.
This should be avoided if possible by use of
digital actuators (such as brushless dc motors
and stepper motors) but there will be cases in
which D/A will be necessary.
It is often a part of A/D conversion
Digital to Analog Conversion




There are different ways of accomplishing D/A
conversion.
The most common method used in simple
converters is based on the ladder network
shown in Figure 11.23.
It consists of a voltage follower. Its input
impedance is high and the output of the follower
equals the voltage at its noninverting input.
The voltage is generated by the resistance
network.
Ladder network D/A conversion
Ladder network D/A conversion



The ladder network is chosen so that the
combination of series and parallel resistances
represent the digital input as a unique voltage
which is then passed to the output.
The switches are digitally controlled analog
switches (MOSFETs).
Depending on the digital input, various switches
will connect resistors in series or in parallel.
Ladder network D/A conversion





For example, suppose that the digital value 100
is to be converted.
The switches will be as in Figure 11.23.
The voltage at the amplifier’s input is exactly 5V.
The ladder can be extended as necessary for
any number of bits.
The accuracy and usefulness of a D/A depends
on the quality and accuracy of the ladder
network and the reference voltage used.
Bridge circuits




Bridge circuits are some of the oldest circuits
used in sensors as well as other applications.
The bridge is known as the Wheatstone bridge
(variations of the bridge exist with different
names.)
The basic Wheatstone bridge is shown in Figure
11.24.
It consists of 4 impedances Zi=Ri+jXi.
The impedance bridge
The impedance bridge

The output voltage of the bridge is
Vo = Vi
Z1
Z3

Z1 + Z2 Z3 + Z4
The bridge is said to be balanced if
Z1 Z3
=
Z2 Z 4
Under this condition, the output voltage is zero.
The impedance bridge




If, for example, Z1 represents the impedance of
a sensor, by proper choice of the other
impedances the output can be set to zero at a
given value of Z1.
Any change in Z1 will change the value of Vo
indicating the change in stimulus.
Of course, one can do much more than that and
bridges can be used for signal translation and
for temperature compensation among other
things.
One important property of bridges is their
sensitivity to change in stimuli
The impedance bridge
The sensitivity of the output voltage to change in
any of the impedances can be calculated as:
dVo = V
Z2
,
i
2
dZ1
Z1 + Z 2
dVo =  V
Z1
i
dZ2
Z1 + Z 2 2
dVo =  V
Z4
,
i
2
dZ3
Z3 + Z4
dVo = V
Z3
i
dZ4
Z3 + Z 4 2
Summing up gives the bridge sensitivity
dVo = Z2dZ1  Z1dZ2  Z4dZ3  Z3dZ4
Vi
Z 1 + Z2 2
Z 3 + Z4 2
The impedance bridge




This relation reveals that if Z1=Z2 and Z3=Z4 the
bridge is balanced
If the change, is such that dZ1=dZ2 and dZ3=dZ4,
the change in output is zero.
This is the basic idea used in compensating a
sensor for temperature variation and any other
common mode effects.
For examples, suppose that a pressure sensor
has impedance Z1=100  and a sensitivity to
temperature dZ1= 0.5 /C.
The impedance bridge






We use two identical sensors as Z1 and as Z2
Sensor Z2 is not exposed to pressure (only
exposed to the same temperature as Z1).
Z3 and Z4 are equal and are made of the same
material – these are simple resistors.
Under these conditions, there will be no output
due to temperature changes
The sensor is properly compensated for
temperature variations.
If however pressure changes, the output
changes
The impedance bridge

If all impedances in the bridge are fixed and only
Z1 varies (this is the sensor), then dZ2=0, dZ3=0,
dZ4=0 and the bridge sensitivity becomes
dVo = Z2dZ1
Vi
Z1 + Z2 2
Or:
dVo dZ1
=
,
Vi
4Z1
if Z 2 = Z1
The impedance bridge

This bridge, especially with resistive branches is
the common method of sensing with:





strain gauges, piezoresistive sensors,
hall elements, thermistors
force sensors and many others.
Use of bridges allows a convenient reference
voltage (nulling), temperature compensation and
other sources of common mode noise.
It is very simple and it can be easily connected
to amplifiers for further processing
Temperature compensation of
bridges




Temperature compensation in sensors
eliminates the errors due to temperature or any
other common mode effect.
It does not eliminate errors external to the
sensors such as variations of Vi with
temperature.
These have to be compensated for in the
construction of the bridge itself.
There are many techniques by which this can be
accomplished but this is beyond the scope of
this course.
Bridge output





The output from the bridge is likely to be
relatively small.
For example, suppose that the bridge is fed with
a 5V source and a thermistor, Z4=500 (at 0C)
is used to sense temperature.
Assuming the bridge is balanced at 0C, the
other three resistances are also 500.
This gives an output voltage zero.
Now, suppose that at 100C the resistance of
the thermistor goes down to 400.
Bridge output

The output voltage now is:
500
400
Vo = 5

= 0.5 V
500 + 500 500 + 500
Most sensors will produce a much smaller change in
impedance
Some sort of amplification will be necessary.
The op-amp discussed above is ideal for this
purpose.
There are many ways this can be accomplished. Two
methods are shown in Figure 11.25.
Amplified bridge
Active bridge
Amplified bridge


In Figure 11.25a, the bridge is connected
directly between the inverting and noninverting
inputs.
If we assume that the resistance of the
resistance of the sensor changes as
Rx=R0(1+), the voltage output of the bridge is:
(1 + n)V
Vout  Vi
4
This circuit provides an amplification of (1+n) but
requires that the voltage on the bridge be floating
Active bridge

Circuit in Figure 11.25b does not provide
amplification but rather places the sensor in the
feedback loop. This is called an active bridge
and its output is:
Vout =  Vi
2
This circuit provides buffering (higher input
impedance, lower output impedance).
Data transmission





Transmission of data from a sensor to the
controller may take many forms.
If the sensor is passive, it already has an output
in a usable form such as voltage or current.
It would seem that it is sufficient to simply
measure this output directly to obtain a reading.
In other cases, such as with capacitive or
inductive sensors, indirect measuring is often
used.
The sensor is often likely to be in a remote
location.
Data transmission



Neither direct measurement of voltage and
current or using the sensor as part of the circuit
(in an oscillator) may be an option in such a
case
In such cases, it is often necessary to process
the sensor’s output locally and to transmit the
result to the controller.
The controller then interprets the data and
places it in a suitable form.
Data transmission





The ideal method of transmission is digital.
Often employed in “smart sensors” since they
have the necessary processing power locally.
In most cases a sensor of this type will have a
local microprocessor supplied with power from
the controller or have its own source of power
The digital data may then be transmitted over
regular lines or even through a wireless link.
Since digital data is much less prone to
corruption, the method is both obvious and very
useful.
Data transmission




Many sensors are analog and,
Their output may eventually be converted
into digital form but:
It is not always possible to incorporate the
electronics locally.
This may be because of cost or because
of operating conditions such as elevated
temperatures.
Data transmission



Example, in a car there may be a half dozen sensors
that control ignition, air intake and fuel, all of which are
needed for control of the engine and are processed by a
central processor.
It is not practical to supply each sensor with power and
electronics to digitize their data when the processor can
do that for all of them.
In other cases, such as, for example, the oxygen sensor,
the sensor operates at elevated temperatures, beyond
the temperature range of semiconductors making it
impossible to incorporate electronics in them.
Data transmission
In such cases the analog signal must be
transferred to the controller.
A number of methods have been developed for
this purpose.
Three of these methods, suitable for use with
resistive sensors, or with passive sensors are
discussed next
Four wire sensing



In sensors that change their resistance, such as
thermistors, and piezoresistive sensors, one
must supply an external source and measure
the voltage across the sensor.
If done remotely, the current may vary with the
resistance of the connecting wires and produce
an erroneous reading.
To avoid this the method in Figure 11.26 may be
used.
Four wire sensing
Four wire sensing






The sensor is supplied from a current source, i0.
This current is constant since the internal
impedance of a current source is very high.
The voltage on the sensor is independent of the
length of the wires and their impedance.
A second pair of wires measures the voltage
across the sensor
Since a voltmeter has very high impedance there
is no current (ideally) in this second pair of wires,
producing accurate reading.
This is a common method of data transmission
when applicable.
Two wire sensing for passive
sensors




Passive sensors produce a voltage. It is sometimes
possible to measure the voltage remotely (no
current is involved in the measurement).
Especially true for dc outputs such as in
thermocouples.
In sensors with high impedance it is much more
risky to do so because of the noise the lines can
introduce.
In most cases a twisted pair line is used because it
reduces the noised picked up by the line.
Two wire transmission for
active sensors





A common method of data transmission for
sensors, and a method that has been
standardized is the 4-20 mA current loop.
The output of the sensor is modified to modulate
the current in the loop
4 mA corresponds to minimum stimulus
20 mA corresponds to maximum stimulus
The configuration is shown in Figure 11.27.
4-20 mA current loop data
transmission
4-20 mA current loop data
transmission



The sensor’s output must be modified to
conform to this industry standard and this may
require additional components.
Many sensors are made to conform to this
standard so that the user only has to connect
them to the two-wire line.
The power supply depends on the load
resistance and the transmitter’s resistance but it
is between 12 and 48V.
4-20 mA current loop data
transmission



Usually the sensor’s network allows for setting
the range (minimum and maximum value of the
stimulus) to the 4 mA and 20 mA range as
shown.
The current transmitted on the line is then
independent of the length of the line and its
resistance.
The voltage measured across the load
resistance is then processed at the controller to
provide the necessary reading.
Other methods of transmission





There are other methods of transmission that may
be incorporated.
6-wire transmission is used with bridge circuits in
which the 4 wire method above is supplemented
by two additional wires which measure the voltage
on the bridge itself.
A new 1-wire protocol has become very popular
for many devices including sensors.
In this protocol both power to the device and data
to/from it are passed on a single pair of wires,
An effective and economical method for sensing.
Transmission to actuators





There are only two ways the power can be
transmitted to the actuator.
One is to get the actuator close to the source
that provides the power.
This implies that lines must be very short.
Possible in some cases (audio speakers, control
motors in a printer, etc.).
In some cases this is not practical and the
controller and the actuator must be at
considerable distance (robots on the factory
floor, etc.).
Transmission to actuators



In such cases one of the methods above may be
used to transfer data but the power must then be
generated locally at the actuator site.
The controller now issues commands as to
power levels, timings, etc. and these are then
executed locally to deliver the power necessary.
Much of this is done digitally through use of
microprocessors on both ends.
Excitation methods and
circuits






Sensors and actuators must often be supplied
with voltages or currents
Either ac or dc.
These are the excitation sources for the sensors
and actuators.
First and foremost is the power supply circuit.
In many sensors the power is supplied by
batteries
Many others rely on line power through use of
regulated or unregulated power supplies.
Excitation methods and
circuits




Other sensors require current sources (for
example - Hall elements)
Still others require ac sources (LVDTs)
These circuits affect the output of the
sensor and its performance (accuracy,
sensitivity, noise, etc.)
Are an integral part of the overall sensor’s
performance.
Power supplies




There are two types of power supplies
Linear power supply
Switching power supply.
There are also so called dc to dc
converters which are used to convert
power from one level to another,
sometimes as part of the circuit that uses
the power.
Power supplies






A linear power supply is shown in Fig. 11.28.
Consists of a source, (line voltage) and a means of
reducing this voltage to the required level ( a transformer).
The transformer is followed by a rectifier which produces
dc voltage from the ac source.
This voltage is filtered and then regulated to the final
required dc voltage. A final filter is usually provided.
This regulated power supply is very common in circuits
especially where the power requirements are low.
Some of the blocks may be eliminated depending on the
application. If, for example the source is a battery the
transformer and the rectifier are not needed and the
filtering may be less important.
Linear regulated power supply
Linear power supply





Consider the circuit in Figure 11.29.
This is a regulated power supply capable of
supplying 5V at up to 1A.
Transformer reduces the input voltage to 16V
rms.
This is rectified through the bridge rectifier and
produces 22V (16x1.4) across C1, C2.
These two capacitors serve as filters – the large
capacitor reducing low frequency fluctuations on
the line, the smaller capacitor is better suited for
high frequency filtering.
Fixed voltage regulated power
supply
Linear power supply





The LM05 is a 5V regulator which essentially
drops across itself 19V to keep the output
constant.
Does so for any input voltage down to about 8V.
The capacitors at the output are again filters.
The current is limited by the capacity of the
regulator to dissipate power due to the current
through it and the voltage across it.
Other regulators are available that will dissipate
more or less power.
Linear power supply



These regulator exist at standard voltages,
either positive or negative as well as adjustable
variable voltage regulators.
Discrete components regulators can be built for
almost any voltage and current requirements.
This circuit or similar circuits are the most
common way of providing regulated dc power to
most sensor and actuator circuits.
Linear power supply
The advantage is that they are simple and
inexpensive but they have serious drawbacks.
 The most obvious is that they are big and heavy,
mostly because of the need for a transformer
which must handle the output power.
 In addition, the power dissipated on the regulator is
not only lost but it generates heat and this heat
must be dissipated through heat exchangers.

Switching power supply

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An alternative method of providing dc power is
through use of a switching power supply.
Switching power supplies rely on two basic
principle to eliminate the drawbacks of the linear
power supply.
The principle is shown in Figure 10.30.
First, the transformer is eliminated and the line
voltage is rectified.
This high voltage dc is filtered as before.
Regulated switching power
supply
Switching power supply
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The switching transistor is driven with a square wave
It turns on for a time ton and off for a time toff
When on, a current flows through the inductor charging
the capacitor to a voltage which depends on ton
When the switch is off, the current in L1 is discharged
through the load supplying it with power for the off-time
The voltage is stabilized by sampling the output and
changing the duty cycle (ratio between ton and toff) to
increase or decrease the output to its required value
This change in duty cycle is done by use of a PWM
(pulse width modulation) generator
Switching power supply
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In a practical power supply additional
considerations must apply.
First, it is necessary to separate or isolate the
input (which is connected to the line) and output.
In the linear PS this was accomplished by the
transformer.
Second, the switching, which must necessarily
be done at relatively high frequencies,
introduces noise into the system.
This noise must be filtered for the PS to be
usable
DC to DC converters
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DC to DC converters are a different type of switching power
supply.
They take the dc source and convert it into an ac voltage
This is then converted through a transformer to any required
level and then rectified back to dc and regulated.
The advantage of this approach is that now the transformer
provides the isolation required for safety
because the operation is at high frequencies, the
transformer is much smaller than the power transformer in
linear power supplies
Transformerless DC-DC converters also common
Current sources
The generation of constant current can take
various levels of complexity.
 One can resort to something as simple as a
large resistor in series with a power supply
 In this configuration the current is not constant
but rather varies because the resistance of the
sensor
 More accurate methods of current generation
are needed for higher accuracy requirements.
Current sources
A simple constant current source can be
built based on the properties of FETs
Shown in Figure 11.31.
As long as the voltage across the FET is
above its pinch-off voltage (Vp), the current
is constant and equals (Vcc-Vp)/R
Vp is constant for any given FET
FET constant current generator
+4 V - 12 V
2N5458 JFET
R
0.001 F
33 F
Current sources
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Another simple way of supplying constant
current to a load is shown in Figure 11.32.
The Zener diode voltage Vz produces a
current in the load equal to (Vz-0.7)/R3
(the voltage across the base-emitter
junction is fixed at 0.7V and the zener
voltage is fixed to Vz).
Zener controlled constant
current generator

R3
R2

RL
Current sources
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A stable circuit is the so-called current
mirror
Current sources
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A current iin is generated as V1/R1 and is kept
constant.
The collector current in the lower left transistor is
virtually equal to iin.
The voltage across the base of Q1 keeps the
current through the load equal to iin, hence the
name current mirror.
As long as iin is constant, so will the current in
the load.
Current sources
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The properties of the voltage follower based on
an op-amp can be used to generate a constant
current as shown in Figure 11.34.
The output of the voltage follower is V1 and the
current is V1/R1.
The transistor is necessary to provide currents
larger than those possible with an op-amp
Voltage follower based constant
current generator
Voltage references
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Many applications call for a constant voltage
reference.
A regulated power supply is a voltage reference
but what is meant here is a constant voltage,
usually of the order of 0.5-2V that supplies very
little current, if any, and is used as reference to
other circuits.
These reference voltages must be constant
under expected fluctuations in power supplies.
Voltage references
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The simplest voltage reference is the Zener diode
Reversed biased diode, biased at the breakdown
voltage for the junction.
The resistor limits this current so that the diode
does not overheat.
As long as the maximum current of the Zener diode
is not exceeded the voltage across the diode is
kept at the breakdown voltage.
These diodes are very commonly used for voltage
regulation and other purposes.
The Zener diode
Reference zener diode
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A Zener diode specifically designed for voltage
reference (called reference Zener diode)
The breakdown voltage is kept constant and
Temperature compensated using two diodes in
series, one forward and one reversed biased
In the forward biased diode, an increase in
temperature decreases the forward voltage (by
V or about 2mV/C)
In the reversed biased diode it decreases it by
roughly the same amount.
The reference Zener diode
Reference zener diode
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The total voltage is constant (or nearly so).
Reference diodes are available in voltages down
to about 3V.
Another device that is used for this purpose is
the band-gap reference.
It is superior to Zener diodes and is available in
voltages that go down to 1.2V.
Reference diodes are available commercially in
standard voltages from about 1.2V to over 100V.
Oscillators
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Many sensors and actuators require voltages or
currents that are variable in time.
Example: the LVDT requires a sinusoidal
sources, often at a few kHz in frequency.
Magnetic proximity sensors use ac currents of
constant amplitude and frequency to produce an
output voltage which is proportional to position.
Transformer based sensors must use an ac
source.
Other sensors require special waveforms such
as square waves.
Oscillators
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Some sensors/actuators use line power (60 or
50Hz),
All other sources must be generated at the
correct frequency and at the required waveform.
Often must be frequency stabilized and
amplitude regulated to make useful sources.
There are virtually hundreds of different ways of
generating as signals but there are a few basic
principles involved.
Oscillators
1.
2
An oscillator is an unstable amplifier.
Starting with an amplifier of some sort, one can
provide a positive feedback to make it unstable
and hence to set it into oscillation.
The unstable circuit must be forced to oscillate
at a specific frequency by means of:
an LC tank circuit (or equivalent) or
a delay in the feedback
The circuit must be made to oscillate with a required
waveform through use of these or additional
components.
Crystal oscillators
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Based on a quartz crystal or other piezoelectric
materials
Cut and placed between two electrodes
The equivalent circuit is an RLC circuit
Can oscillate in one of two modes.
One is a series oscillation mode,
The other is parallel mode oscillation
When connected in a circuit that can provide the
proper positive feedback, it will oscillate at the
resonant frequency of the crystal
Structure of a crystal
Equivalent circuit of a crystal
A 1 MHz crystal
Sinusoidal crystal oscillator
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Simple sinusoidal oscillator
The feedback from output to input (collector to
base) is supplied by the crystal.
The output is entirely defined by the crystal and
is taken at the collector.
The trimmer capacitor modifies the equivalent
circuit.
Sinusoidal crystal oscillator
Square wave crystal oscillator
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Based on two inverting gates
Because the gate can only take two states, the
output will swing between Vcc and ground.
The positive feedback is delayed due to the
delay of the gate and the frequency is controlled
by the crystal.
These oscillators can be used, for example, in
mass humidity sensors in which the frequency
will change with humidity (mass of the crystal).
TTl based square wave crystal
oscillator
RC Oscillators
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Oscillators can easily
be built from discrete
as well as integrated
components without a
crystal.
A simple square wave
oscillators based on the
delay of the feedback
signal (RC) is shown
next
RC oscillators
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The inverters are triggered when the input
voltage rises above about Vcc/2.
Resistor R and capacitor C form a charging
circuit.
Suppose left gate is on (zero input, Vcc output).
The second gate must be off (its output is zero)
Lower capacitor charges (time constant RC) and
after a time t0 triggers left gate to change state.
Now its output is zero and the capacitor
discharges through R. The upper capacitor is
only needed for stability of the circuit.
RC oscillators
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The following circuit is somewhat similar.
RC oscillator
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Positive feedback through R3 sets the level at
which the amplifier changes state.
R4 and C1 form the charging/discharging circuit.
Suppose that Vout is high. The positive input will
be set at a value that depends on R3, R2 and R1.
C1 charges through R4.
When the voltage at the negative input exceeds
that at the positive input the output goes
negative
Now the capacitor discharges through R4,
repeating the process.
LC oscillator
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Examples of sinusoidal oscillators
An LC circuit is provided which oscillates
at the required frequency
A feedback is provided from output to input
The feedback is through the lower part of
L1 or through the lower half of the LVDT
coil (figures)
Sinusoidal LC oscillator
Sinusoidal LC oscillator
Noise and interference
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Noise is understood as anything that is not
part of the required signal.
Many sources and many types of noise.
We will distinguish between two broad
types
Inherent noise to the sensor (internal).
Interference noise (external).
Inherent noise
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Noise must be reduced as much as possible
elimination is not an option since noise cannot
be entirely eliminated
More important is to properly consider it in the
design and in the specification of the sensor.
Example: a temperature sensor generates 10
V/C and a good microvolt meter is capable of
reliably measuring 1 V.
This, would imply a resolution of 0.1C.
Inherent noise
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Suppose noise (from all sources) is, say, 2 V
Only signals above the noise levels are useful
Any signal below 2 V is useless.
The resolution cannot be more than 0.2 C.
In many cases, things are worse than this since
the noise can only be estimated.
When amplification occurs, noise is also
amplified and the amplifier itself can add its own
noise.
Clearly then noise cannot be ignored even when
it is small.
Inherent noise
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Inherent noise is due to many effects in the sensor
Some of the sources are avoidable,
Some of the sources are intrinsic.
One of the main sources in sensors is the thermal noise
or Johnson noise in resistive devices.
The noise power density is usually written as:
en2
= 4kTRf
V2
Hz
k is the Boltzman constant (k=1.38x10-23 J/K),
T is the temperature in K,
R is the resistance in 
f is the bandwidth in Hz.
Inherent noise
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This noise exists, in resistive sensors and
in simple resistors
Ff the resistance is high, the noise can be
very high.
The Johnson noise is fairly constant over a
wide range of frequencies
Hence it is called a white noise
Inherent noise
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Shot noise:
Produced in semiconductors when dc current
flows by random collisions of electrons and
atoms:
isn = 5.7104 If
Preference is for lower currents in as much as this
noise is concerned.
Inherent noise
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Pink noise:
Unlike white noise has higher energy at low
frequencies.
A particular problem with sensors which tend to
operate at low frequencies (slowly varying
signals).
The noise spectral density is 1/f and at low
frequencies it may be larger than all other
sources of noise.
Inherent noise
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Noise levels are very difficult to measure even
when the noise is constant.
Because it is not generally harmonic in nature,
its rms or even peak to peak values are difficult
to ascertain.
The noise distribution is not constant (usually
Gaussian) so that at best we can estimate the
noise level.
Usually maximum expected levels are indicated.
Interference
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By far the largest source of noise in a sensor or
actuator
Originates outside the sensor and is coupled to
it.
Sources of interference can be many:
Best known perhaps are the electric sources:
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coupling of transients from power supplies,
electrostatic discharges
radio frequency noise from all electromagnetic
radiative systems (transmitters, power lines, almost
all devices and instruments that carry ac currents,
lightning and even from extraterrestrial sources).
Interference
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Interference can be mechanical
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Thermal sources (
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Vibrations
gravitational forces
acceleration and others,
temperature variations
Seebeck effect in conductors
Also: ionization sources, errors due to changes
in humidity and even chemical sources.
Interference
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Some errors are introduced in the layout of the
sensors components or in the circuits connected
to them through improper circuit design and
improper use of materials.
Electrical sources of noise are called
electromagnetic sources (including static
discharges and lightning)
Are bundled together under the umbrella of
electromagnetic interference or electromagnetic
compatibility issues.
Interference
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In some cases, a noise is easily identifiable.
Example: a common noise in electrical system,
especially those that contain long wires, is a
120Hz noise (100 Hz in 50Hz power systems)
and is due power lines.
This type of noise is also a good example of a
time-periodic noise.
Other sources, especially when transient or
random are almost impossible to identify and
hence to correct.
Interference
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Interference noise may affect different sensors
differently.
The simplest is an additive influence.
That is, the noise is added to the signal.
Additive noise is independent of the signal.
Additive noise is more critical at low signal levels
Example: drift due temperature variations
depends on temperature but not on the signal
level.
This type of noise can be minimized by using a
differential sensor
Interference
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A second type of noise is multiplicative.
That is, it grows with the signal and is due to a
modulation effect of the noise on the signal.
More pronounced at higher signal levels.
The noise may be minimized by using two
sensors as previously the output is divided by
the reference sensors’ output.
Example: a stimulus is measured (say, pressure)
and a noise due to change in temperature T is
present and multiplicative.
Interference
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Assume the transfer function is V=(1 + N)Vs
One sensor senses both the stimulus and the
noise and produces an output V1 which is:
V1 = [1 + T]Vs
The second sensor senses only the temperature and
produces a voltage V2
V2 = [1 + T]V0
V0 can be assumed constant (i.e. it is only
dependent on temperature change)
Interference
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The ratio between the two is:
V1 Vs
=
V2 V0
Since V0 is independent of the sensed stimulus, the
ratio is also independent of the noise.
This is called a ratiometric method and is most
suitable for this type of noise
Interference
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Reduction of noise before it reaches the sensor.
Most important is electrical noise
Electrical noise can reach the sensor in four
ways
through direct resistive coupling
Through capacitive coupling
Through inductive coupling
By radiation from outside the sensor
Interference - Resistive coupling
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Source of noise and the sensor share a common
resistive path.
May be the resistance between the connection
of a sensor, through the sensor’s body.
That is, the sensor is not electrically insulated
from the source of noise.
Solution: isolation of the sources of noise
(usually current carrying conductors such as
power lines) from the sensor.
Often this will require that the sensor be floating.
Interference - capacitive
coupling
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Capacitance exists between any two conductors,
Any two wires, any two connectors will produce
a stray capacitance that can cause coupling.
Capacitances are small - impedances are high.
Capacitive coupling is a problem at higher
frequencies.
There are however sensors, especially
capacitive sensors which use small
capacitances
Any capacitive coupling may be too high for
accurate sensing.
Interference - capacitive
coupling
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Solution: the sensor must be electrostatically
shielded from the sources that might couple
noise.
An electrostatic shield is usually a thin
conducting sheet, sometimes a conducting
mesh, which envelopes the protected area and
is grounded (connected to the reference
potential.
In effect this shorts the noise source to ground.
An example is shown in Figure 11.45.
Electrostatic shielding
Interference - capacitive
coupling
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The coupling capacitance is shorted
This also creates a new capacitance between
the protected device and ground.
But, the noise signal is zero.
Cables leading to the sensor must also be
shielded
The shield must be at a constant potential.
Example: shielding a cable and then grounding it
at both ends, will immediately produce a loop
which may itself generate noise.
Interference - inductive coupling
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A particular problem between current carrying
conductors
Example: between power lines and sensors’
conductors and in particular the wires leading to
the sensor.
120 Hz noise from power liner usually links to
sensors through inductive coupling
Actuators may induce currents in sensors
Sensors may interfere with each other
Interference - inductive coupling
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At high frequencies, a conducting shield just like
the electrostatic shield should envelope the source.
The use of coaxial cables is such an example.
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Based on the idea of skin depth (Chapter 9) and simply
takes advantage of attenuation of high frequency fields in
conductors.
If the noise signal is very low in frequency, a
magnetic shield is necessary.
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Usually a thick ferromagnetic shield (box) that envelopes
the protected device to guide low frequency (or DC)
fields away from the sensor. Proximity sensors often use
this type of shield.
Interference
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Together, conduction, capacitance and
inductance form a class of coupling called
conductive coupling and is part of the common
problem of conducted emission and
conducted interference in electromagnetic
compatibility.
Interference - radiated emission
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Any conductor carrying an ac current is in effect
a transmitting antenna.
Any other conductor becomes a receiving
antenna.
If that conductor is part of a loop, a current will
be induced in the loop.
This noise is particularly large from sources of
intentional emissions such as transmitters
Can occur with any current, internal or external
to the sensor.
Interference - radiated emission
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Reduction of this source relies extensively on
reduction of lengths of wires and on reduction of
size (area) of loops.
Shielding is very effective in reducing radiated
interference.
Other precautions: use of decoupling capacitors
in circuits and power supplies
Twisting of the two wires leading to a device
together to reduce the area of the loop they
form.
Interference - radiated emission
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Coaxial cables can reduce or eliminate most
radiated interference.
One common cure for many ills is the
introduction of a ground plane – a sheet of metal
under the circuit (such as a conducting sheet
under a printed circuit board).
This helps in reducing the inductance of the
circuit and hence will be effective in reducing
both inductive coupling and radiated
interference.
Mechanical noise
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Mechanical noise, especially from vibrations can
often be eliminated or reduced through isolation
Some sensors, such as piezoelectric sensors,
any force (due to acceleration) will produce
errors
These errors can be compensated either
through use of the differential or ratiometric
methods
Many other sources of noise
Other sources of noise
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Example: any junction between different metals
becomes a thermocouple and introduces a
signal in the path.
This may affect the reading of the sensor and is
called Seebeck noise.
It may not be a big problem in most cases but it
is when sensing temperature.
The issue of noise is both difficult and ill-defined.
Often finding the source of noise will depend on
sleuthing work and on experimentation.