of Microprocessors

378 MUSICAL ApPLICATIONS OF MICROPROCESSORS
are associated with the lesser significant bits because, after all, their influence
on the output is less.
If the DAC is offset binary encoded, the largest glitch, therefore, occurs
right at zero crossing. Unfortunately, even low-amplitude signals are going
to cross zero so the distortion due to glitching can become absolutely intolerable
at low signal levels. The distortion also becomes worse at higher frequencies,
since more glitch energy per unit time (power) is being released.
Sign-magnitude coding can have the same problem because of switching
transients from the sign-bit amplifier. Floating-point DACs, however, attenuate
the zero-crossing glitch along with the signal at low levels.
Glitch magnitude and duration are not often specified on a DAC data
sheet. When they are, the units are very likely to be volt-seconds because of
the unpredictable nature of DAC glitches. Unfortunately, this alone is insufficient
information to even estimate the distortion that might be produced in
an audio application. What is needed is the rms voltage of the glitch and its
duration so that the energy content can be determined.
Low-Glitch DAC Circuits
The two primary causes of glitching in DAC circuits are differences in
the switching time of the various bits (skew) and differences between the bit
turn-on and turn-off times. In some DAC circuits, the most significant bit
switches carry considerably more current than the least significant bits, thus
contributing to differences in switching times. Some R-2R designs, however,
pass the same current through all of the bit switches, thus eliminating
this cause of skew. Even the digital input register can contribute to skew,
since it will undoubtedly span two or more ICs that may have different
propagation times. The use of high-speed Schottky registers, which are
verified to have equal delay times, will minimize this source of skew. Any
sign-magnitude or floating-point translation logic should be in front of the
input register as well.
Nonsymmetrical turn-on/turn-off time is an accepted fact of life among
TTI logic designers. The reason is storage time in the saturated bipolar
transistor switches, which also applies to many DAC analog switch designs.
The digital input registers often accentuate the problem for the same reason.
There are, however, low-glitch DACs on the market that use emittercoupled
logic internally for the register and nonsaturating current steering
analog switches.
Typically, these are only available in I2-bit versions, since they are
designed primarily for CRT deflection circuits. One type that is available has
a maximum glitch amplitude of 40 mY, a duration of 60 nsec, and a
full-scale output of ±5 V. With a I-kHz full amplitude output, the glitch
distortion would be about-78 dB with respect to the signal, about 6 dB
below its I2-bit quantization noise. At 10 kHz, however, the glitch distortion
rises by 10 dB, making it the dominant noise source.