Biomedical Engineering Reference
In-Depth Information
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FIGURE 9.38
(Top) Second-order Butterworth low-pass filter. (Bottom) Third-order Butterworth low-pass
filter.
continuous-time waveform into a digital signal (i.e., the amplitude takes one of 2
n
discrete
values), which is converted into computer words and stored in computer memory. To
adequately capture the continuous-time signal, the sampling instants
must be selected
carefully so information is not lost. The minimum sampling rate is twice the highest
frequency content of the signal (based on the sampling theorem from communication
theory). Realistically, we often sample at five to ten times the highest frequency content
of the signal so as to achieve better accuracy by reducing aliasing error.
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9.14.1 Noise
Measurement signals are always corrupted by noise in a biomedical instrumentation sys-
tem. Interference noise occurs when unwanted signals are introduced into the system by
outside sources, such as power lines and transmitted radio and television electromagnetic
waves. This kind of noise is effectively reduced by careful attention to the circuit's wiring
configuration to minimize coupling effects.
Interference noise is introduced by power lines (50 or 60 Hz), fluorescent lights, AM/FM
radio broadcasts, computer clock oscillators, laboratory equipment, and cellular phones.
