Biomedical Engineering Reference
In-Depth Information
ANALOG
SIGNAL
CONDITIONER
DATA
ACQUISITION
SYSTEM
DATA
STORAGE AND
DISPLAY
SENSORS
DIGITAL
SIGNAL
PROCESSING
PARAMETER
TO BE
OBSERVED
FIGURE 11.4
Sensors adapt the signal that is being observed into an electrical analog signal that can be measured
with a data acquisition system. The data acquisition system converts the analog signal into a calibrated digital signal
that can be stored. Digital signal processing techniques are applied to the stored signal to reduce noise and extract
additional information that can improve understanding of the physiological meaning of the original parameter.
After the biosignal has been detected with an appropriate sensor, it is usually amplified
and filtered. Operational amplifiers are electronic circuits that are used primarily to
increase the amplitude or size of a biosignal. Bioelectric signals, for instance, are often faint
and require up to a thousand-fold boosting of their amplitude with such amplifiers. An
analog filter may then be used to remove noise or to compensate for distortions caused
by the sensor. Amplification and filtering of the biosignal may also be necessary to meet
the hardware specifications of the data acquisition system. Continuous signals may need
to be limited to a certain band of frequencies before the signal can be digitized with an
analog-to-digital converter, prior to storing in a digital computer.
11.4.3 A/D Conversion
Analog-to-digital (A/D) converters are used to transform biological signals from contin-
uous analog waveforms to digital sequences. An A/D converter is a computer-controlled
voltmeter, which measures an input analog signal and gives a numeric representation of
the signal as its output. Figure 11.5a shows an analog signal, and Figure 11.5b shows a dig-
ital version of the same signal. The analog waveform, originally detected by the sensor and
subsequently amplified and filtered, is a continuous signal. The A/D converter transforms
the continuous, analog signal into a discrete, digital signal. The discrete signal consists of a
sequence of numbers that can easily be stored and processed on a digital computer. A/D
conversion is particularly important because storage and analysis of biosignals are becom-
ing increasingly computer based.
The digital conversion of an analog biological signal does not produce an exact replica
of the original signal. The discrete, digital signal is a digital approximation of the original,
analog signal that is generated by repeatedly sampling the amplitude level of the original
signal at fixed time intervals. As a result, the original, analog signal is represented as a
sequence of numbers: the digital signal.
