of Microprocessors

412 MUSICAL ApPLICATIONS OF MICROPROCESSORS
DIGITAL
r---r---7 OUTPUT
Fig. 12-30. Audio ADC
successive approximation must be reasonably fast in order to attain the higher
audio sample rates. For example, the settling time of the DAC plus the
response time of the comparator must be less than 2 f-Lsec if 12-bit
conversions are to be done at 40 ks/s. This can be a very strict requirement if
a fast sample rate and high resolution for audio recording/playback is desired.
The successsive approximation logic is as described in Chapter 7. The FIFO
buffer holds the last sample value converted until the host can accept it. As
with the DAC, the buffer may be as simple as a single register or a true
multilevel hardware FIFO. Uniformity of sample rate is just as important as
before and therefore should come from a crystal-controlled timing generator.
The sample-and-hold, however, performs a completely different function.
Rather than gating out glitches, its job is to capture the input signal at
an instant in time and hold it long enough to be digitized. Slewing and
glitching are not important, but aperture time and aperture uncertainty are
critical for low noise and distortion in the converted results.
Figure 12-31 illustrates the job to be done by the SAH module.
Acquisition time specifies the interval between the sample command and the
time when the SAH is adequately following the signal. This time usually
varies with signal level but need not cause concern unless it becomes longer
than the sample period minus the ADC conversion time. Even when the
signal is being followed, there is a finite lag in the tracking called tracking
error. As long as the lag time is constant, its only effect is a very slight
reduction in high-frequency response, but if it varies with signal amplitude,
distortion can be introduced.
When the hold command is given, there is a finite delay before the
sampling switch starts to turn off, which is called aperture delay. Once the
switch begins to turn off, there is an additional delay before it is completely
turned off, which is called aperture time, and is a critical audio SAH parame-