Download Circuit Design and Examples

Circuit Design and
Examples
•Design Guidelines
–Objectives
–Sensor
–Signal Conditioning
•Example
•Instrumentation Amplifiers
–AD620
•OpAmps
–Op07 (modern)
–LM741 (ancient)
1
Design Guidelines-1
• Define the measurement objective
– Parameter: What do you need to measure:
pressure, temperature, flow, level, etc
– Range: What is the range of the
measurements? 50-1500 F, 25-65 psi, 100200 volts
– Accuracy: What accuracy is desired and what
specification of accuracy will be used? 5% of
Full Scale or 2% of the reading.
– Linearity: Must the measurement be linear?
– Noise: How much noise is allowed?
2
Sensor Definition in Engineering: the component of an instrument that converts
an input signal into a quantity that is measured by another part of the instrument
and changed into a useful signal for an information-gathering system.
A transducer is an electronic device that converts energy from one form to
another. Common examples include microphones, loudspeakers, thermometers,
position and pressure sensors, and antenna. Although not generally thought of as
transducers, photocells, LEDs (light-emitting diodes), and even common light
bulbs are transducers.
Design Guidelines-2
• Select the sensor/transducer
– Parameters: What is the input and output of the transducer?
E.g. pressure in resistance out, temperature in voltage out,
light in current out
– Transfer Function: Output/input relationship?
– Time Response: 1st order, 2nd order?
– Range: What is the possible range of sensor parameters? 01000C, 3-15 psi, etc
– Power: What is the power specification of the sensor?
3
Design Guidelines-3
• Analog Signal Conditioning
– Parameter of output? Voltage, current,
pressure, frequency
– Range? 0-5V, 3-15psi, 4-20mA, 300-3500 Hz
– Input impedance of the signal conditioning
circuit? Many sensors require a specific
impedance input or a range of allowable
inputs
– Output impedance to the next stage?
4
Example: Problem 2.33 in text
My solution (check the solution
manual for the author’s solution)
2.33: A bridge circuit
has R1=R2=R3=120
ohms and V=10.0
volts. Design a signal
conditioning system
that provides an
output of 0 to 5 volts
as R4 varies from 120
to 140 ohms. Plot Vout
vs R4. Evaluate the
linearity
Desired output is VA-B
A
B
5
1. Place a normal resistor as R4.
2. Double click on the value of the
resistance
3. Enter {Rvar} [Yes the braces are
required]
4. Go to “Get New Part” in the Draw Menu
5. Place the part name “PARAM” on the
schematic page
6. Double click on Parameters
7. Enter Rvar as Name1
How to create
a graph with
a varying
resistance
And Value1=100
8. Go to Analysis/Setup menu
9. Check and open the DC
Sweep box
10.Check Global Parameter and
Linear
11.Enter the Name of the variable
Which is Rvar here,
the start and end values of
the sweep and the increment
for the sweep
12. Simulate
10
A
B
6
A
B
R3
R4
VA  VB  10  (

)
R1  R3 R 2  R 4
VA-VB
7
VA-VB
0 to .3846 to 0 to 5
Requires a gain of
5/.3846=13.0
A
B
8
Need Differential Gain of 13
9
Instrumentation Amplifiers
Analog Devices Inc. is the largest supplier of
instrumentation amplifiers in the world.
The AD620 is a low cost, high accuracy instrumentation amplifier which requires
only one external resistor to set gains of 1 to 1000. Furthermore, the AD620 offers
lower power (only 1.3 mA max supply current), making it a good fit for battery
powered, portable (or remote) applications.
The AD620, with its high accuracy of 40 ppm maximum nonlinearity, low offset
voltage of 50 µV max and offset drift of 0.6 µV/°C max, is ideal for use in precision
data acquisition systems, such as weigh scales and transducer interfaces. The
low noise, low input bias current, and low power of the AD620 also make it well
suited for medical applications such as ECG and noninvasive blood pressure
monitors.
The low input bias current of 1.0 nA max is made possible with the use of
Superbeta processing in the input stage. The AD620 works well as a preamplifier
due to its low input voltage noise of 9 nV/Hz at 1 kHz, 0.28 µV p-p in the 0.1 Hz to
10 Hz band, 0.1 pA/µHz input current noise. The AD620 is also well suited for
multiplexed applications with its settling time of 15 µs to 0.01% and its cost is low
enough to enable designs with one in amp per channel.
10
AD620
Specifications
common-mode rejection ratio
(CMRR): The ratio of the
common-mode interference
voltage at the input of a circuit, to
the corresponding interference
voltage at the output.
http://products.analog.com/products/info.asp?product=AD620
11
Electrostatic Warning for the
AD620 In-Amp
12
Make vs. Buy: A Typical Bridge Application Error Budget
The AD620 offers improved performance over “homebrew” three
op amp IA designs, along with smaller size, fewer components and
lower supply current. In the typical application, a gain of 100 is required
to amplify a bridge output of 20 mV full scale over the industrial
temperature range of –40°C to +85°C.
Regardless of the system in which it is being used, the AD620
provides greater accuracy, and at low power and price.
Note that for the homebrew circuit, the OP07 specifications for
input voltage offset and noise have been multiplied by 2, because a
three op amp type in-amp has two op amps at its inputs.
AD620 vs opamp
13
Error Budget
14
• The OP-07 has very low input offset voltage (25µV max for OP07A) which is obtained by trimming at the wafer stage. These low
offset voltages generally eliminate any need for external nulling.
The OP-07 also features low input bias current (±2nA for OP-07A)
and high open-loop gain (300V/mV for the OP-07A). The low
offsets and high open-loop gain make the OP-07 particularly
useful for high-gain instrumentation applications.
• The wide input voltage range of ±13V minimum combined with
the high CMRR of 110dB (OP-07A) and high input impedance
provides high accuracy in the non-inverting circuit configuration.
Excellent linearity and gain accuracy can be maintained even at
high closed-loop gains.
• The OP-07 is available in five standard performance grades.
The LM741 series are general purpose operational amplifiers which
feature improved performance over industry standards like the LM709.
They are direct, plug-in replacements for the 709C, LM201, MC1439 and
748 in most applications.
Op07 vs LM741
15
Op07 vs 741
$1.25 for one Op07
$0.44 for one LM741
25 for $25
25 for $8
From Digikey
(Inexpensive
versions of each)
Input Offset
Voltage
Input Offset
Current
CMRR
Op07 (Analog
Devices)
30 to 75 uV
LM741 (National
Instruments)
6 to 7.5 mV
.4 to 2.8 nA
200 to 300 nA
110 dB Min
70 dB Min
Closed Loop BW
(gain = 1)
Slew Rate
.6 MHz
.437 MHz
.3 V/uSec
.5 V/uSec
16
Summary
• Design Guidelines
– Objectives
– Sensor
– Signal Conditioning
• Example
• Instrumentation Amplifiers
– AD620
• OpAmps
– Op07 (modern) from Texas Instruments, Linear
Technology, or Maxim
– LM741 (old but useful) from National Semiconductor
17
Digital Signal Conditioning
•AC Bridges
•Number systems
•Boolean Algebra Example
•Tristate Buffers
•Comparators and Circuits
•Schmidt Trigger
•Window Detector
18
AC Bridge Circuits
Generalized AC Bridge
A
B
Balanced when:
Z1Zx = Z2Z3
19
R3
1
R1 ( Rx 
)  R2 (
)
j C x
1  jC3 R3
Condition for Balanced Bridge
R1
R2
C3
R3
Wien Bridge
Rx
Cx
20
Wien Bridge Oscillator Circuit
http://niuhep.physics.niu.edu/~eads/phys475/lab9.html
.001 uF
10KΩ
10KΩ
.001 uF
Expected Sine Wave
Frequency=15.9 KHz
Adjust the 50K resistor
for a sine wave output
21
http://chem.ch.huji.ac.il/~eugeniik/instruments/test/bridges_theory.html
AC Bridges
22
Number Systems
http://www.ibilce.unesp.br/courseware/datas/data1.htm
BITS
A bit is the smallest element of information used by a computer.
A bit holds ONE of TWO possible values: 0 meaning
Off/False/NotSet and 1 meaning On/True/Set
Boolean Values
Boolean algebra recognizes True and False. So a single bit can
represent a Boolean variable.
NIBBLE
A nibble is a group of FOUR bits. This gives a maximum
number of 16 possible different values. 2 ^ 4 = 16
LSB and MSB:
The Least Significant Bit (LSB) is always drawn at the extreme
right and has the least value and the Most Significant Bit (MSB)
is always shown on the extreme left, and is the bit with the
greatest value.
23
BYTES
Bytes are a grouping of 8 bits. This comprises TWO nibbles.
Binary Coded Decimal [BCD]
Binary code decimal digits (0-9) are represented using FOUR
bits. The valid combinations of bits and their respective values
are 0000 through 1001 with the binary combinations 1010 to
1111 not used.
If the computer stores one BCD digit per byte, its called normal
BCD. The unused nibble may be all 0's or all 1's.
Packed BCD:
If two BCD digits are stored per byte, its called Packed BCD.
This occurs in data transmission where numbers are being
transmitted over a communications link. Packed BCD reduces
the amount of time spent transmitting the numbers, as each
data byte transmitted results in the sending of two BCD digits.
Number Systems
24
Number Systems
Hexadecimal
Refers to the base-16 number system, which consists of 16
unique symbols: the numbers 0 to 9 and the letters A to
F. e.g. decimal 15 is represented as F in hexadecimal.
This is useful because it can represent a byte (8 bits) as
two hexadecimal digits. It is easier to read hexadecimal
numbers than binary numbers.
To convert a value from hexadecimal to binary, translate
each hexadecimal digit into its 4-bit binary equivalent.
Hexadecimal numbers have either an 0x prefix or an h
suffix. For example, the hexadecimal number 0x3F7A
translates to the following binary number: 0011 1111
0111 1010
http://www.webopedia.com
25
Multiplication/Division
• Multiplication by 10: Shifting left in decimal
multiplies by 10. E.g. 05010
50010
• Multiplication by 2: Shifting left in binary
multiplies by 2. E.g. 01002
10002
which translates to 410
810
• Division works the same way in that
shifting right divides by 10 in decimal, 2 in
binary, 8 in octal, and 16 in hexadecimal.
26
Push-On Push-Off Control Circuit
http://www.oldradio.com/current/pushon-pushoff.htm
N.C.
24 volt dc
N.O.
Relay Coil
Push-Button
Switch
27
A
B
C
D
0
0
0
0
0
0
0
1
0
0
1
0
0
0
1
1
0
1
0
0
0
1
0
1
0
1
1
0
0
1
1
1
Example
Wt Sensor IR Sensor Robot Welder Vision
A
B
C
System
D
A +B
1
0
0
0
1
0
0
1
1
0
1
0
1
0
1
1
1
1
0
0
C
1
1
0
1
D
1
1
1
0
1
1
1
1
AB
A
B
Nand
ABCD
Nand
CD
C +D
28
Tri-state Buffers
http://www.websdeveloped.com/Billy/ComputerOrganization/Assignments/2/Assignment2.htm
Enable
1
1
29
Comparators
Maxim
http://para.maxim-ic.com/Comparators.htm
National Semiconductor
http://www.national.com/catalog/AnalogComparators.html
Texas Instruments
http://amplifier.ti.com
Fairchild
Fairchild Semiconductor
30
National Semiconductor
LM111 Comparator:
(LM311 is $0.52 each at
Digikey)
http://www.national.com/pf/LM/LM111.html
http://www.brouhaha.com/~eric/pic/open_drain.html
Open-drain outputs are outputs which at
any given time are either actively sinking
current (i.e., low voltage, typically
considered logic 0) or are high impedance,
but which never source current (high
voltage, logic 1). Open-drain refers to the
drain terminal of a MOS FET transistor. The
equivalent concept on a bipolar device is
called open-collector.
Article on the meaning of Rail-to-rail
http://www.chipcenter.com/images/tn026.pdf
31
http://home.cogeco.ca/~rpaisley4/Comparators.html
A
B
Comparator Circuit
32
LM339 is $0.52 at Digikey
http://www.national.com/ads-cgi/viewer.pl/ds/LM/LM139.pdf
Zero Crossing Detector
33
100 Hz Sinusoid
2.5 +2.5Sin(2π*100*t)
Schmidt
Trigger
Circuit
34
Switches Low
Schmidt Trigger
Switches High
35
http://webug.physics.uiuc.edu/courses/phys344/344exp/Lab1Analog_Digital/Archive/Su2000/lab1.htm#Part_5_Window_Comparator
Window Detector
36
Summary
•
•
•
•
•
•
•
AC Bridges
Number systems
Boolean Algebra Example
Tristate Buffers
Comparators and Circuits
Schmidt Trigger
Window Detector
37