SIGNAL CONDITIONINGUsually the sens

SIGNAL CONDITIONING

Usually the sensor output cannot be directly coupled to the display device. Simple signal conditioners may only amplify and filter the signal or merely match the impedance of the sensor to the display. Often sensor outputs are converted to digital form and then processed by specialized digital circuits or a microcomputer (Tompkins and Webster, 1981). For example, signal filtering may reduce undesirable sensor signals. It may also average repetitive signals to reduce noise, or it may convert information from the time domain to the frequency domain.


OUTPUT DISPLAY

The results of the measurement process must be displayed in a form that the human operator can perceive. The best form for the display may be numerical or graphical, discrete or continuous, permanent or temporary—depending on



1 . 3 A L T E R N A T I V E O P E R A T I O N A L M O D E S 7


the particular measurand and how the operator will use the information. Although most displays rely on our visual sense, some information (Doppler ultrasonic signals, for example) is best perceived by other senses (here, the auditory sense). User controls and the output display should conform to the Human Factors Engineering Guidelines and Preferred Practices for the Design of Medical Devices (AAMI, 1993).


AUXILIARY ELEMENTS

A calibration signal with the properties of the measurand should be applied to the sensor input or as early in the signal-processing chain as possible. Many forms of control and feedback may be required to elicit the measurand, to adjust the sensor and signal conditioner, and to direct the flow of output for display, storage, or transmission. Control and feedback may be automatic or manual. Data may be stored briefly to meet the requirements of signal conditioning or to enable the operator to examine data that precede alarm conditions. Alternatively, data may be stored before signal conditioning, so that different processing schemes can be utilized. Conventional principles of communications can often be used to transmit data to remote displays at nurses’ stations, medical centers, or medical data-processing facilities.


1.3 ALTERNATIVE OPERATIONAL MODES DIRECT-INDIRECT MODES

Often the desired measurand can be interfaced directly to a sensor because the measurand is readily accessible or because acceptable invasive procedures are available. When the desired measurand is not accessible, we can use either another measurand that bears a known relation to the desired one or some form of energy or material that interacts with the desired measurand to generate a new measurand that is accessible. Examples include cardiac output (volume of blood pumped per minute by the heart), determined from measurements of respiration and blood gas concentration or from dye dilution; morphology of internal organs, determined from x-ray shadows; and pulmonary volumes, determined from variations in thoracic impedance plethysmography.


SAMPLING AND CONTINUOUS MODES

Some measurands—such as body temperature and ion concentrations— change so slowly that they may be sampled infrequently. Other quantities— such as the electrocardiogram and respiratory gas flow—may require continuous monitoring. The frequency content of the measurand, the objective of the measurement, the condition of the patient, and the potential liability of



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the physician all influence how often medical data are acquired. Many data that are collected may go unused.


GENERATING AND MODULATING SENSORS

Generating sensors produce their signal output from energy taken directly from the measurand, whereas modulating sensors use the measurand to alter the flow of energy from an external source in a way that affects the output of the sensor. For example, a photovoltaic cell is a generating sensor because it provides an output voltage related to its irradiation, without any additional external energy source. However, a photoconductive cell is a modulating sensor; to measure its change in resistance with irradiation, we must apply external energy to the sensor.


ANALOG AND DIGITAL MODES

Signals that carry measurement information are either analog, meaning continuous and able to take on any value within the dynamic range, or digital, meaning discrete and able to take on only a finite number of different values. Most currently available sensors operate in the analog mode, al- though some inherently digital measuring devices have been developed. Increased use of digital signal processing has required concurrent use of analog-to-digital and digital-to-analog converters to interface computers with analog sensors and analog display devices. Researchers have developed indirect digital sensors that use analog primary sensing elements and digital variable-conversion elements (optical shaft encoders). Also quasi-digital sensors, such as quartz-crystal thermometers, give outputs with variable frequency, pulse rate, or pulse duration that are easily converted to digital signals.
The advantages of the digital mode of operation include greater accuracy,
repeatability, reliability, and immunity to noise. Furthermore, periodic cali- bration is usually not required. Digital numerical displays are replacing many analog meter movements because of their greater accuracy and readability. Many clinicians, however, prefer analog displays when they are determining whether a physiological variable is within certain limits and when they are looking at a parameter that can change quickly, such as beat-to-beat heart rate. In the latter case, digital displays often change numbers so quickly that they are very difficult and annoying to observe.


REAL-TIME AND DELAYED-TIME MODES

Of course sensors must acquire signals in real time as the signals actually occur. The output of the measurement system may not display the result immediately, however, because some types of signal processing, such as averaging and transformations, need considerable input before any results can be produced. Often such short delays are acceptable unless urgent