Text 8. Differential Amplifier

Most amplifiers deal with signals which exist between a signal wire and ground. Differential amplifiers, which are widely used in ECG, EEG, EMG, and pressure amplifiers are intended to amplify the difference in voltage between two signal lines and should ignore any voltage which exists simultaneously on the two signal lines (the common mode signal). The most common common-mode signal is 50 Hz (or 60 Hz) pick-up which may be several volts in some situations, whereas the differential signal may be measured in microvolts in the case of the EEG. Thus a good differential amplifier must have a high common mode rejection ratio (CMRR) of the order of 80 to 100 dB.

CMRR is degraded if the source impedance is different on each signal lead (such as in the case of skin electrodes with unequal quality of application) and so CMRR is usually quoted for a particular source impedance inequality (e.g. 5000 [O])

Text 9. Transducer

This is a device for the conversion of one form of energy to another. In medical work the best known examples are temperature, pressure, ion concentration, force and displacement transducers. The term could be applied to any energy-converting device such as a motor or light bulb but it is usually reserved for an electrically operated measuring device or actuator.

1. Pressure transducers. The electrical versions of these usually work by allowing the pressure to bend a diaphragm in the transducer and measure the bending by resistance strain gauges arranged into a bridge circuit which may be attached to the diaphragm (as in the case of most semiconductor gauges) or they may be remote from the diaphragm (unbounded) as in the case of wire or metallic film gauges. Such devices are used extensively for recording of blood pressure (via intra-arterial or intravenous cannulae), bladder pressure, etc.

2. Temperature transducers. The thermoelectric potential between two dissimilar metals (thermocouple), or the change in resistance due to temperature (thermistor) may be used to measure and record temperatures.

3. Displacement transducers. Distance moved, or position, may be measured by a potentiometer (e.g. as used to determine the position and orientation of some ultrasonic B-scan transducers) or the ratio of inductive coupling between two coils may be varied by the movement of a third coil or ferrite core (LVDT). Displacement transducers may also be used as velocity or acceleration transducers with suitable electronics.

4. Ultrasonic transducers. Sound or ultrasound may be produced by an inductive device such as the loudspeaker or magnetostrictive transducer, or be a piezoelectric element which expands or contracts according to the applied voltage. The advantage of the latter for medical applications is that they can work at high frequencies ( e.g. 5 MHz) and are reciprocal in that they can convert acoustic energy into electrical signals as well as the reverse.

Text 10. Pressure Transducer

Physiological pressures range from about 18 kPa (140 mmHg) for arterial blood pressure to 0.26 kPa (2mmHg) for airways pressure. These and other pressures may be measured and recorded for diagnostic or monitoring purposes. Because the mercury manometer is used so much for blood pressure recording it is very common to quote physiological pressure in millimetres of mercury (mmHg) rather than in SI units (Pa - pascals). Also some pressures such as bladder pressure and central venous pressure are often quoted in centimetres of water head. This is particularly convenient with water-filled measurement tubing since changes in the height of the transducer or patient make the same changes in the measured pressure. Useful conversions are as follows:

1 mmHg = 133 Pa = 0.133 kPa

1 mmHg = 1.36 cmH2O

1 cmH2O = 0.735 mmHg

Although static pressures can be measured using a column of mercury (e.g. the mercury sphygmomanometer) or water, changing pressures require a pressure transducer which can respond to rapid changes. These usually work by producing an electrical signal from the bending of a membrane or diaphragm.

The strain gauge which measures the bending may be part of the pressure diaphragm itself as in semiconductor gauges or it may be attached to the diaphragm (usually a thin steel plate) by a rod. The strain gauges change their electrical resistance as the diaphragm is bent by the pressure.

The gauges form part of a bridge circuit which provides a voltage output corresponding to the applied pressure. A differential amplifier is required to amplify this signal before delivery to a CRT monitor or pen recorder. Inductive, capacitance, and piezoelectric strain gauges also exist.

Physiological pressure transducers are usually 2-3 cm in diameter and are mounted in a clamp close to the patient and connected to the site of measurement (artery, heart, bladder, lungs etc.) by a narrow tube which is usually filled with water or saline. Special miniature versions also exist which are mounted on the tip of a narrow tube which can be passed directly into the body.

Favourable characteristics for a pressure transducer are high-frequency response, low drift with temperature and time, low compliance, and electrical isolation.