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 RX 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 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 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 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 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 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 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 measure- ment 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 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 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.
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