Page 78 - Concise Encyclopedia of Robotics

Data Conversion
Analog to digital
Any analog, or continuously variable, signal can be converted into a
string of pulses whose amplitudes have a finite number of states. This is
analog-to-digital (A/D) conversion.
An A/D converter or ADC samples the instantaneous amplitude of an
analog signal and outputs pulses having discrete levels, as shown in Fig.
1. The number of levels is called the sampling resolution, and is usually a
power of 2. The number of pulses per second is the sampling rate. The
time between pulses is the sampling interval. In this example, there are
eight levels, represented by three-digit binary numbers from 000 to 111.
111
110
Analog waveform
101
100
011
010
001

000
Time
Sampling interval

Data conversion—Fig. 1

In general, the minimum workable digital sampling rate is approxi-
mately twice the highest analog data frequency. This is a general principle
in communications engineering, known as the Nyquist theorem or sam-
pling theorem. For a signal with components as high as 3 kHz, the min-
imum sampling rate is 6 kHz. The commercial voice standard is 8 kHz.
For hi-fi music transmission, the standard sampling rate is 44.1 kHz. In
machine communications systems, the minimum sampling rate de-
pends on the speed with which data must be transferred between
points, such as from a central controller to a mobile robot.

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