ENTC 4350

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ENTC 4350

• MEDICAL INSTRUMENTATION TRANSDUCERS
• AND AMPLIFIERS

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• 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

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• 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.

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SIGNAL
TRANSDUCER
DETECTOR
AMPLIFIER
RECORDER
The figure shows the fundamental principles of
the process of measuring physical data by means
of electrical signals.
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SIGNAL
TRANSDUCER
DETECTOR
AMPLIFIER
RECORDER
The transducer may be any device that converts
physical energy into an electrical signal.
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SIGNAL
TRANSDUCER
DETECTOR
AMPLIFIER
RECORDER
The interface is simply whatever connects or lies
between the transducer and the patient.
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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.
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SIGNAL
TRANSDUCER
DETECTOR
AMPLIFIER
RECORDER
The amplifier amplifies the signal for the
recorder, and the recorder records or stores the
data.
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• 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.

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• Physiological parameters include
• An extremely weak voltage,
• A pressure,
• A fluid flow rate,
• A temperature,
• A chemical concentration, or
• An electrolyte level.

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• 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

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• CONVERSION OF PHYSIOLOGICAL PARAMETERS
• INTO VOLTAGES

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• 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.

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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.

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• 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.

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• Likewise, the physiological parameters of
  temperature and pressure are converted to a
  machine-measurable parametervoltageby a
  balanced structure.
• In this case, it is a balanced circuit called a
  Wheatstone bridge.

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• Wheatstone Bridge

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• 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.

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• 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.

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• 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.

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• 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 .

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• The voltage difference between the two nodes is
  defined as the output voltage, VOUT, so
• 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.

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• 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.

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• Thus, VA becomes less than VB and VOUT becomes a
  negative voltage.

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• 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.

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• 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

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• 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.

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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.

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• 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.

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• 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.

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• Commonly used thermistor elements are made from
  oxides of nickel, copper, or aluminum.
• This gives the thermistor elements a relatively
  high temperature coefficient.

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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.

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• 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.

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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.

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• 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.

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• A commonly used pressure transducer is shown.
• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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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.

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• 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.

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VOLTAGE AMPLIFIERS

• Amplifiers are as old as history.
• A lever with a fulcrum for prying up stone is a
  force amplifier.

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• 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.

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• Notice that the output force is in the opposite
  direction from the input force.
• This is an example of an inverting amplifier.

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• A pressure amplifier is illustrated.
• It consists of two disks attached to either end
  of a rod.

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• 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.

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• 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.

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• The tympanic membrane and the oval window of the
  inner ear form a pressure amplifier of this type.

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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.

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• 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.

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• 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.

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• 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.

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• 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.

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• The ideal diff amp is an elegant and powerful
  concept.
• It helps explain a large number of medical
  instrumentation principles.

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• 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
• where AD is the gain of the amplifier.

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• 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.

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• The gain, AD, is the ratio of the output voltage
  to the difference between the two input voltages.
  
• It is a dimensionless number.

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• 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.

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• 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.

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• 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
• and the output resistance approaches zero.

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• This means that the load placed on the output of
  the amplifier will not change the value of the
  output, VOUT.

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• 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.

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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.

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• 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.

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• The situation in making a difference measure-ment
  on the body is shown.
• This illustrates the basic problem of such a
  measurement in the hospital environmentpower
  line, 60-cycle interference.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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
  60-cycle voltage, but it passes the difference
  potentials under test.

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• Real world diff amps are not ideal, so they do
  not perfectly reject common-mode 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.

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• This CMRR is often given in decibels (dB) and
  would have a value in excess of 100 dB in a
  useful diff amp.

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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.

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• A block diagram of the thermometer is shown.

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• 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.

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• 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.

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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.

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• The elements of a pressure monitor are shown.

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• 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.

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• An actual monitor in use in the hospital would
  have many other features to ensure reliability,
  ease of use, accuracy, safety, and convenience.