of Microprocessors

DIGITAL-TO-ANALOG AND ANALOG-TO-DIGITAL CONVERSION 413
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Fig. 12-31. SAH errors
ter. It is critical because the partially off switch is highly nonlinear and
contributes to distortion. Aperture uncertainty is a variation in aperture delay
usually caused by variations in signal amplitude. It has the same distortioncausing
effect as variation in the sample rate. Both of these aperture errors
create distortion proportional to frequency and amplitude. Essentially, then,
the aperture is the effective time width over which the signal voltage is
measured and, much like a camera lens, a small aperture gives sharper focus
over a wider range of conditions.
Hold step usually is not important unless it varies with signal level.
One must wait until the turn-off transient decays before starting the conversion
cycle, however. Hold droop is almost never a problem, since the hold
time is a few dozen microseconds at most.
Often, it is helpful to have an automatic gain-control (AGC) circuit in
the signal path before the SAH and ADC to keep signal levels consistently
high. In order to retain information about the dynamics of the input, it
would be a simple matter to digitize the gain-control voltage in the AGC
circuit with an inexpensive 8-bit ADC module. Since this voltage changes
slowly, its sample rate could be as low as tens of hertz. Internally, the AGC
information could multiply the samples from, say, a l2-bit ADC into full
l6-bit samples and restore the dynamic range. Note that the response speed
of the AGC circuit has no bearing on the accuracy of reconstruction provided
it is fast enough to suppress sudden, high-amplitude transients below the
ADC clipping point.