Download V OUT - Faculty

ENTC 4350
MEDICAL
INSTRUMENTATION
TRANSDUCERS
AND AMPLIFIERS

Although the measurement of physical
parameters like force and pressure are
rarely of medical interest in themselves,
the determination of these parameters
underlay a vast variety of medical
techniques.
• Cardiac
• pulmonary function

To make a measurement, we must have
something to measure.
• Force and pressure are often difficult to
measure directly and accurately.
• We therefore measure these data indirectly by
converting them into an electrical signal, which can
be filtered, amplified, recorded, etc.
SIGNAL
TRANSDUCER
DETECTOR
AMPLIFIER
RECORDER
The figure shows the fundamental principles of the process of
measuring physical data by means of electrical signals.
SIGNAL
TRANSDUCER
DETECTOR
AMPLIFIER
RECORDER
The transducer may be any device that converts physical energy
into an electrical signal.
SIGNAL
TRANSDUCER
DETECTOR
AMPLIFIER
RECORDER
The interface is simply whatever connects or lies between the
transducer and the patient.
SIGNAL
TRANSDUCER
DETECTOR
AMPLIFIER
RECORDER
The detector is any device used to pick out the electrical signal we
want to measure. Not all transducers require a detector.
SIGNAL
TRANSDUCER
DETECTOR
AMPLIFIER
RECORDER
The amplifier amplifies the signal for the recorder, and the recorder
records or stores the data.

In most cases, the function of the
transducer is to convert a physiological
parameter into a voltage that is large
enough to be processed accurately by
the electronic equipment.

Physiological parameters include
• An extremely weak voltage,
• A pressure,
• A fluid flow rate,
• A temperature,
• A chemical concentration, or
• An electrolyte level.


To perform this task, the transducer must
be properly placed on the patient, as well
as strategically placed into an electronic
circuit, such as a Wheatstone bridge.
Trans
CONVERSION OF
PHYSIOLOGICAL
PARAMETERS
INTO VOLTAGES

Three of the most commonly measured
physiological parameters in health care
are
• temperature,
• blood pressure, and
• weight.
• All of these may be measured by means of a
balanced structure, such as a scale.
Consider how a scale works.

Before the patient steps on it, the scale
is in balance, and it reads zero.
• Another way of saying this is that the scale
pointer is on a null.
• The patient on the scale throws it out of balance,
causing a displacement of the pointer, which is
calibrated in pounds.

In this case, the physiological parameter
of weight is transformed to a
displacement of a pointer.
• Here, the transducer is the platform the
patient stands on, and the structure of the
balance is the arrangement of levers and
springs in the scale.

Likewise, the physiological parameters
of temperature and pressure are
converted to a machine-measurable
parameter—voltage—by a balanced
structure.
• In this case, it is a balanced circuit called a
Wheatstone bridge.
Wheatstone Bridge

The Wheatstone bridge,
which consists of four
resistors arranged in a
diamond shape and labeled
R1, R1, R3, and Rx.
•
An excitation voltage, VE , is
applied to two points of the
diamond, and an output
voltage, VOUT, is measured
plus to minus from left to right
across the other two points of
the diamond.

The two resistors on the left, Rx and R1,
form a voltage divider of the VE
excitation.
• This produces the plus-to-minus voltage drop
from node A to ground, VA.

Likewise, the two resistors on the right,
R2 and R3, form a voltage divider that
creates the voltage drop from node B to
ground, VB.

This circuit can be made balanced, in the
simplest case, by making all four
resistors the same value.
• In this case, the voltage divider on the left
creates the same voltage as that on the right,
because they both have the same excitation
voltage and the same resistor values.
• Thus, VA equals VB .

The voltage difference between the two
nodes is defined as the output voltage,
VOUT, so
VOUT  VA  VB
• In this case, VOUT is zero, and the bridge is
said to be at a null point in terms of its
resistance values.
• That is, the bridge is balanced.

This bridge can be made unbalanced by
changing the value of Rx .
• If Rx is caused to increase, the voltage divider
on the left will cause VA to decrease in value.
• Because the divider on the right is undisturbed, VB
will remain the same.

Thus, VA becomes less than VB and VOUT
becomes a negative voltage.

On the other hand, if Rx is caused to
decrease from its null value, VOUT will
become a positive voltage drop from
node A to node B.
• As an exercise, prove that to yourself by
studying the figure.

You have learned the case where the
bridge is balanced because all resistors
have the same value.
• In fact, the bridge can be balanced for any
number of resistor value combinations given
by the formula
R3
R X  R1
R2

This equation is called the null condition
for the bridge.
• If Rx is increased above the value given by
this equation, VOUT will leave zero and be a
negative voltage.
• And if Rx is decreased from its null value, VOUT will
become positive.
Thermistor

A thermistor is a transducer that makes it
possible to convert the physiological
parameter of temperature into a voltage.
• A thermistor may be constructed of a cube of
material, about 0.1 inch on a side, embedded
in glass whose electrical resistance varies
with its temperature.
• Almost all electrical conductors exhibit this property
to some degree.

For example, if copper is heated, the
atoms will vibrate harder, making it more
difficult for free electrons to get past
without a collision.
• This increases its resistance.
• Thus, copper has a positive temperature
coefficient, because an increase in temperature
causes an increase in resistance.

Some metals act similarly, but in the
opposite direction.
• For example, an increase in temperature in a
semiconducting metal like silicon will break
more electrons free from their crystal bonds
and increase the number of free electrons, so
that an increase in temperature will decrease
the resistance.
• Because of this, silicon is said to have a negative
temperature coefficient.

Commonly used thermistor elements are
made from oxides of nickel, copper, or
aluminum.
• This gives the thermistor elements a relatively
high temperature coefficient.
Temperature Transducer

A thermistor mounted in a Wheatstone
bridge can function as the transducer
that converts body temperature to a
voltage.
• This may be used as the transducer for an
electronic thermometer.
• Its advantage over the traditional mercury
thermometer is its fast response time and ease of
reading, not to mention the fact that mercury from a
broken thermometer is a hazardous material.

In a blood donor screening, for example,
reducing the three minutes it takes to do
a temperature with a mercury
thermometer becomes important.
• On the other hand, the electronic
thermometer is more complicated, bulkier,
and may not last as long as the mercury
thermometer.
Pressure Transducer

Blood pressure is most commonly
measured with an air cuff and
stethoscope using a device called a
sphygmomanometer.
• This is the noninvasive test given in a blood
donor screening.

For intensive care situations, however, it
may be necessary to use an invasive
procedure.
• Here, the focus is on how the physiological
parameter of pressure is transformed into a
voltage.

A commonly used pressure transducer is
shown.
Diaphragm
A
B
Armature
C
D
Strain-gage wires
•
The dome on the top may be filled with a saline
solution that articulates to a catheter, as in the heart to
measure the blood pressure in a ventricle.
• The other fluid coupling connection is blocked off.

Changes in blood pressure propagate
through the catheter and cause small
displacements in the diaphragm.
• These displacements move a plunger to
which are connected four wires, called strain
gauges.

With each displacement, two of these
wires lengthen and the other two get
shorter.
• Lengthening the wire increases its resistance,
while shortening the wire decreases its
resistance by the same amount.

Lengthening a wire causes it to increase
in resistance both because it gets longer
and because its cross-sectional area
reduces.
• These high resistance wires are arranged in
the form of a Wheatstone bridge.

In the figure, each of the strain gauge
wires is represented by a resistor, R,
plus a change in resistance, DR,
imposed by changes in pressure on the
diaphragm.
• Notice on the left branch of the bridge that a
positive DR increases the upper resistance
and decreases the lower resistance.

Thus, VA would decrease.
• Because of the change in sign of the DRs on
the right branch, VB would go in the opposite
direction and increase.
• The net result is that VOUT, defined as plus to minus
from node A to node B, would be a negative
voltage.

If the pressure on the diaphragm
changes to the opposite direction, VOUT
would become a positive voltage.
• Thus, you have a mechanism that converts
the pressure changes into voltage changes.
• This voltage could be used to drive electrical
meters and monitoring equipment.
Pressure Transducer Sensitivity

In general, the sensitivity of a pressure
transducer, SV, is defined as the change
in output voltage per volt of excitation
per millimeter of mercury of applied
pressure (V/V/mmHg).
• A typical commercially available pressure
transducer has a sensitivity ranging from 5
mV/V/mmHg to 40 mV/V/mmHg, depending
upon the manufacturer and model.

Some disposable pressure transducers
work on the same electrical principle just
described.
• The manufacturing process for these
transducers is inexpensive enough that the
unit can be disposed of rather than put
through an expensive sterilization process.
• In fact, in some cases, trying to sterilize a
disposable unit can damage it and make it
inaccurate.
VOLTAGE AMPLIFIERS

Amplifiers are as old as history.
• A lever with a fulcrum for prying up stone is a
force amplifier.

A force down on one side of the lever will
cause a larger force going in the
opposite direction to be exerted on the
other side of the lever.
• The closer the fulcrum is to what is being
pried up, the larger that force will be.

Notice that the output force is in the
opposite direction from the input force.
• This is an example of an inverting amplifier.

A pressure amplifier is
illustrated.
•
It consists of two disks
attached to either end of a
rod.

If a pressure is exerted on the larger disk
in the direction shown in the figure, the
smaller disk will exert a larger pressure
in the same direction.
• For example, if PIN on the disk on the left is 1
pound per square foot on a 1-square-foot
area, the rod will transmit that 1 pound to the
smaller disk at a pressure of 1 pound per
square inch.
• This converts to a pressure of 144 pounds per
square foot.

This, therefore, is an example of a
pressure amplifier with a gain of 144.
• In this case, the output pressure, POUT is in
the same direction as PIN.
• This is an example of a noninverting amplifier.

The tympanic membrane and the oval
window of the inner ear form a pressure
amplifier of this type.
Differential Amplifier

The surface potentials that are
measured on the body for medical
diagnosis, such as
• The electrocardiogram (ECG),
• The electroencephalogram (EEG), and
• The electromyogram (EMG),
are all difference potentials.

A difference potential is that voltage
measured between two sites on the
body.
• For example, the EGG measured between
two wrists is a difference potential.

The amplifier for measuring difference
potentials is called a differential amplifier.
• To make a differential amplifier, electronic
transistors are arranged in the form of a
Wheatstone bridge.

A differential amplifier, often abbreviated as diff
amp, is an electronic amplifier in which the
output voltage is proportional to the difference
between two input voltages.
•
Diff amps are particularly useful for measuring
biopotentials, because many biopotentials of clinical
and medical diagnostic significance consist of the
difference in voltage on two body sites.

The EEG is the difference in surface
potential between two skull sites.
• Likewise, the EMG records the difference
between two potentials measured on a
muscle.
• The diff amp is ideal for measuring these difference
potentials and is often used in medical
instrumentation.

The ideal diff amp is an elegant and
powerful concept.
• It helps explain a large number of medical
instrumentation principles.

A diff amp is defined as an electronic
amplifier in which the output voltage,
VOUT, is proportional to the difference
between the two input voltages, V1 and
V2.
• This definition can be written mathematically
as
VOUT  AD V2  V1 
• where AD is the gain of the amplifier.

The diff amp is illustrated.
•
•
V1 measured from minus to
ground from the upper input
node, is the inverting input
voltage.
V2 measured to ground from the
lower input node, is the
noninverting input voltage.

The gain, AD, is the ratio of the output
voltage to the difference between the
two input voltages.
• It is a dimensionless number.

This will be considered an ideal diff amp
when the resistance at each input node
is very large (more than 40 megohms).
• This means that essentially zero current will
flow into either of the input nodes.

Another implication is that attaching the
input leads of the diff amp to another
circuit will not disturb that circuit in any
way.
• In measuring body surface potentials, for
example, this would imply that attaching the
amplifier to the sites measured would not
• Distort those voltages,
• Introduce artifacts, or
• Attenuate them.

In other words, the ideal diff amp is
“invisible” to the parameter it measures.
• In the ideal diff amp, the VOUT measured to
ground is given by
VOUT  AD V2  V1 
• and the output resistance approaches zero.

This means that the load placed on the
output of the amplifier will not change the
value of the output, VOUT.

In the previous equation, notice that
when the input voltages, V1 and V2, are
the same (or common-mode), the output
voltage is zero.
• This is what is meant when a diff amp is said
to reject common-mode voltage.
• In other words, the output due to a common-mode
voltage at the inputs is zero in an ideal diff amp.
Common-Mode Voltage
Interference

The importance of diff amps is
heightened by the fact that one of the
major tasks in monitoring, diagnosing,
and making measurements on medical
patients is the measurement of
difference potentials that occur in the
body;
• That is, the EGG, EEG, or EMG.

They are all measured as differences
between sites on the surface of the body.
• In each case, the instrument for doing this is
the diff amp.

The situation in making
a difference measurement on the body is
shown.
•
This illustrates the basic
problem of such a
measurement in the
hospital environment—
power line, 60-cycle
interference.

In such an environment, where
thousands of pieces of electrical
equipment are in use, the power
requirements are high.
• Inevitably, patients are in close proximity to
power buses through stray capacity between
them and their bodies, which are essentially
conductors.

The amount of capacity is in the order of
10 pF (10 x 1012 farad).
• This value varies widely with the situation, but
it should give you a feeling for how much
capacity is involved.
• This capacity couples a current into the patient and
generates a voltage on the input terminals V1 and
V2 in the previous figure.

The value of the voltages is the same on
both terminals because the body is all
one conductor.
• Therefore, the voltages are common-mode
voltages.

A common-mode voltage is one that has
the same value over the entire surface of
the body.
• The value of the voltages is about 2 volts at
60 cycles.

You can measure these voltages on an
oscilloscope by simply holding onto the
conducting end of the input lead.
• They are much larger in size than the body
potential voltages of an EGG, which is about
1 mV.

Because they are common-mode
voltages fed to a diff amp, the diff amp
output due to them is ideally zero.
• However, the output due to the EGG will be
whatever its difference value is at the input
multiplied by the gain, AD.
• That is, the diff amp rejects the common-mode 60cycle voltage, but it passes the difference
potentials under test.

Real world diff amps are not ideal, so
they do not perfectly reject commonmode voltage interference.
• For them, the common-mode rejection ratio
(CMRR) is defined as the ratio of the VOUT
due to a voltage when presented to the
amplifier as a common-mode signal to the
VOUT due to the same signal presented as a
difference voltage.

This CMRR is often given in decibels
(dB) and would have a value in excess
of 100 dB in a useful diff amp.
Electronic Thermometer

A simple example of how the diff amp is
used in a medical instrument is as a
component of an electronic thermometer.
• The temperature transducer defined
previously can be used along with a diff amp
to make such a thermometer.

A block diagram of the
thermometer is shown.

In order to have an understanding ot this
device, or any medical instrument for that
matter, it is important to be able to follow the
information variables through the device,
beginning with the physiological parameter
under test and ending with the output display
data.
•
In the figure, temperature, T, is applied to the
thermometer.

The temperature changes the resistance in the
thermistors in the bridge.
•
•
•
This determines the value of the voltage difference
between nodes (connections) A and B.
These nodes are wired to the diff amp, the output of
which is proportional to the difference voltage.
That voltage then drives the display on the scale
where a number corresponding to the temperature
appears.
Pressure Monitor

A pressure monitor uses a diff amp in a
similar fashion.
• In both cases, it responds to the voltage
developed across the output of a Wheatstone
bridge and drives a display.

The elements of a
pressure monitor are
shown.

The path of the information variables of
pressure, P, and voltage through the
instrument is as follows:
•
•
•
The pressure from the fluid catheter in the blood
vessel is exerted on the pressure-sensitive resistors in
the Wheatstone bridge.
The difference voltage from nodes A to B that results
is wired to the diff amp, which produces a voltage
output proportional to it.
The output from the diff amp drives the display unit,
which gives a reading of the pressure.

An actual monitor in use in the hospital
would have many other features to
ensure reliability, ease of use, accuracy,
safety, and convenience.