Transducers by E2YT05


									 ENTC 4350

   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
    • 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.


The figure shows the fundamental principles of the process of
measuring physical data by means of electrical signals.


The transducer may be any device that converts physical energy
into an electrical signal.


The interface is simply whatever connects or lies between the
transducer and the patient.


The detector is any device used to pick out the electrical signal we
want to measure. Not all transducers require a detector.


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
   Trans-
   Three of the most commonly measured
    physiological parameters in health care
    • 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
    • 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
    • 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
                   RX  R1
   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.
   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 resis-tance.
   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
    • 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
    • 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
Pressure Transducer
   Blood pressure is most commonly
    measured with an air cuff and
    stethoscope using a device called a
    • This is the noninvasive test given in a blood
      donor screening.
   For intensive care situations, however, it
    may be necessary to use an invasive
    • Here, the focus is on how the physiological
      parameter of pressure is transformed into a
   A commonly used pressure transducer is
    shown.                     Diaphragm




                                                          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 ventri-cle.
         • 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
   With each displacement, two of these
    wires lengthen and the other two get
    • 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
    • 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
    • Notice on the left branch of the bridge that a
      positive DR increases the upper resistance
      and decreases the lower re-sistance.
   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
   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
    • 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
   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
    •   It consists of two disks
        attached to either end of a
   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
    • For example, the EGG measured between
      two wrists is a difference potential.
   The amplifier for measuring difference
    potentials is called a differential
    • 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
       • The diff amp is ideal for measuring these difference
        potentials and is often used in medical
   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
    • This definition can be written mathematically
                 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
    •   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
    • 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
   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
    • That is, the EGG, EEG, or EMG.
   They are all measured as differences
    between sites on the surface of the
    • In each case, the instrument for doing this is
      the diff amp.
   The situation in making
    a difference measure-
    ment on the body is
    •   This illustrates the basic
        problem of such a
        measurement in the
        hospital environment—
        power line, 60-cycle
   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
   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
   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 60-
        cycle voltage, but it passes the difference
        potentials under test.
   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.
   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
    • 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
    •   In the figure, temperature, T, is applied to the
   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
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
   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.

To top