of Microprocessors

DIGITAL-TO-ANALOG AND ANALOG-TO-DIGITAL CONVERSION 373
3-BIT
GAIN
CONTROL
DIGITAL
GAIN
CONTROL
12-BIT
DAC
ANALOG
SIGNAL
GAIN-CONTROLLED
AMPLIFIER
OUTPUT
Fig. 12--4. Floating-point DAC
16 bits for quite some time, and the difference would probably be inaudible
anyway.
Segmented DAC
The sign-magnitude DAC just -described is really a special case of the
segmented DAC discussed in Chapter 7 with two segments: one from minus
full scale to just under zero and the other from just over zero to plus full scale.
Use ofa real segmented DAC with the usual 8 or 16 segments in fact provides
the same benefits as sign magnitude coding with the advantage of having the
code conversion logic and sign-bit stage built into the DAC module itself
There are in fact 16-bit DACs on the market designed for audio applications
that use the segmented architecture. Note that, although the differential
linearity at zero (and elsewhere) may be essentially perfect, if the spec sheet
reads 13-bit integral linearity (0.006%), then at high signal levels the
distortion due to that nonlinearity will be the same as a 13-bit DAC. In
practice, the distortion generated by a segmented DAC with 13-bit integral
linearity will be mostly low-order harmonics that tend to be less audible than
the sharp spikes caused by localized nonlinearities in an equivalent all-ladder
DAC. Interestingly, a harmonic distortion analyzer's typical rms readout
would lead one to the opposite conclusion.
Floating-Point DACs
A method commonly used in analog audio equipment to improve
apparent SiN ratio is to "ride the gain." Strong signals are made stronger by
increasing the gain, while weak signals are made even weaker by reducing
the gain. Since the noise is most audible at low signal levels, the act of
reducing the gain also reduces the noise and so improves the SiN ratio. The
main problem encountered in this scheme is knowing when and how much to
change the gain setting.
In an audio DAC application, it should be possible to apply the same
concept to a 12-bit DAC to improve its SiN ratio at low signal levels.
Consider the setup in Fig. 12-4. Twelve-bit samples operate the 12-bit DAC
in the normal way. However, three additional bits control a variable-gain