of Microprocessors

DIGITAL-TO-ANALOG AND ANALOG-TO-DIGITAL CONVERSION 379
Sample-and-Hold Deglitcher
The usual solution to DAC glitching at the sample rates used in audio
applications is the incorporation of a sample-and-hold module at the output
of the DAC. In operation, it would be switched to the hold state just prior to
loading the next sample into the DAC and would not be switched back into
the track mode until the DAC has settled at the new level. In this way,
glitches from the DAC are not allowed to reach the output.
As a practical matter, however, even SAH modules glitch when
switched from one state to another. This is alright, however, if the magnitude
and polarity of the glitches are constant and independent of the signal
level. In that case, the glitches contain energy only at the sample rate and its
harmonics, which will eventually be filtered out. Most commercial SAH
modules are fairly good in this respect. A linear variation of glitch magnitude
with the signal level can also be acceptable, since the only effect then would
be a slight shift in the dc level of the output.
Another SAH parameter that must be constant for low distortion is the
switching time from hold to sample mode. If this varies nonlinearly with
signal voltage level, then harmonic distortion is produced. Unfortunately,
the switching time of most analog switches is signal voltage dependent. The
reason is that they cannot turn on (or off) until the switch driver voltage
crosses the switch threshold voltage which, as was discussed in Chapter 7, is
relative to the signal voltage. Since the driver voltage does not have a zero
rise time, the time to cross the threshold voltage will vary with the signal
level. A linear variation would be alright, but a perfectly linear drive voltage
ramp is unlikely. However, if the drive voltage is of sufficiently large
amplitude and centered with respect to the signal, reasonably linear variation
can be obtained.
Slew-Limiting Distortion
There still exists a subtle yet quite significant distortion mechanism in
the typical SAH module that is unrelated to glitching or switching time
variation. The mechanism is slew limiting of the amplifiers in the SAH
module when switching from hold mode to sample mode. Figure 12-7 shows
a typical SAH module. When the sampling switch is closed, the circuit acts
as a simple voltage follower. When open, A2 buffers the capacitor voltage
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Fig. 12-7. Typical feedback SAH circuit