Biomedical Engineering Reference
In-Depth Information
5.5
DIGITAL SIGNAL PROCESSING
5.5.1 The Comparator
The comparator is the simplest analog-to-digital converter (ADC), as it converts an analog
signal with an infinite number of amplitudes to a single-bit digital output. If the analog
signal is above a given threshold the output is high, and if it is lower the output is low, as
shown in Figure 5-40.
In reality, the analog signal almost always includes a small amount of noise, so as
it nears the threshold level multiple triggering may occur. This can be eliminated by
including some positive feedback from the comparator output to add hysteresis to the
threshold level. This circuit configuration is referred to as a Schmitt trigger.
5.5.2 Signal Acquisition and Processing Overview
Processing signals within a computer (digital signal processing) requires that they be
sampled periodically and then converted to a digital representation using an ADC. As a
result, the continuous signal is reduced to being defined only at discrete points, as shown
in Figure 5-41. The sampling theorem (also known as Shannon's sampling theorem) states
that to ensure accurate representation the signal must be sampled at the Nyquist rate, which
is defined as at least double the highest significant frequency component of the signal.
f
s
≤
2
f
max
(5.27)
In addition, the number of discrete levels to which the signal is quantized must also be
sufficient to represent variations in the amplitude to the required accuracy. Most ADCs
quantize to 12 or 16 bits, which represent 2
12
=
4096 or 2
16
=
65536 discrete levels,
respectively.
FIGURE 5-40
Comparator circuit.
FIGURE 5-41
Digitizing a signal.
