of Microprocessors

DIGITAL-TO-ANALOG AND ANALOG-TO-DIGITAL CONVERSION 411
DAC WR PULSE ~ IOpsec F
~_&--------,
DATA INTO DAC ::X~ .....JX"- _
~----
DAC OUTPUT
DEGLITCH SWITCH O~~ 11..-__..1
FILTER INPUT -------..- '-....
N~ATIVE EXPONENTIAL CURVES~ ----
Fig. 12-29. Audio DAC timing diagram
been latched from the data-in lines and that a new sample should be made
available. The data could be supplied directly by a running audio-output
program, direct memory access circuit, or FIFO buffer logic. The DAC
output signal is 10 V peak-to-peak in amplitude but is reduced to 5 V by the
filter compensation stage. The filter itself reduces it further to 2.5 V but then
is amplified back to 10 V (3.5 V'lns) by the output amplifier, a level sufficient
to overcome minor ground-loop noise. Ground-loop concerns may be
completely eliminated in exchange for dc response by incorporating a
standard 600-ohm line transformer into the unit (use a 25-JLF nonpolarized
dc blocking capacitor in series with the transformer primary). The output
amplifier can drive a primary impedance as low as 600 ohms.
Only three adjustments are necessary after the unit is built. For best
linearity in the DAC, the offset voltage ofAl and A2 must be nulled. Do this
by removing the 7546, temporarily shorting both amplifier inputs to
ground, and turning the offset null pot to the point where the amplifier
output switches between - 14 and + 14 V. The dc offset at the final output is
zeroed with the potentiometer in the 7546 reference circuit. Perform the
adjustment with the DAC running and outputting a string of zero samples.
This is important because control feedthrough capacitance in the deglitcher
contributes to the output offset.
Au.dio Digitizing
At this time, let us take a brief look at analog-to-digital conversion of
audio. In a synthesis application, the major reason for digitizing audio is for
modification or source-signal analysis. Consequently, there is not as much
need for superdynamic range and distortion figures as with the audio DAC.
Like the DAC, an off-the-shelf ADC alone is not suitable for audio. A
block diagram of an audio ADC is shown in Fig. 12-30. The low-pass filter
considerations are not the same as those with the DAC except that high
sample rates do llOt eliminate the need for a filter. The DAC used for