of Microprocessors

234 MUSICAL ApPLICATIONS OF MICROPROCESSORS
MOST
SIGNIFICANT
3 BITS
{
MSB
7
6
5
4
3
2
I
o
TO CORRESPONDING
ANALOG SWITCHES
LEAST
SIGNIFICANT
BITS
~
V,efh
LADDER
DAC
Vout
V,efl
Fig. 7-9. Segmented DAC
The split can be made anywhere, but the incentive is to apply most of the
bits to the ladder. Commercial units typically apply the most significant 3 or
4 bits to the resistor string DAC. Other circuit structures using current
division and interpolation are also possible.
The main advantage of this circuit is much improved differential
linearity for a given level of resistor accuracy. For example, a 16-bit
segmented DAC using 16 segments can be guaranteed monotonic using
resistors in the ladder section that would otherwise only be precise enough for
a 12-bit unit. This is a 16-fold relaxation in accuracy requirement that much
more than pays for the increased circuit complexity over what a straight 16­
bit ladder-type DAC would need. However, as with the original resistor
string DAC, the integral linearity is no better than the resistor accuracy in the
string section, so such a DAC might only have 12-bit equivalent accuracy. In
applications in which step-to-step uniformity is more important than slight
curvature of the overall transfer function, segmented DACs can be very
valuable. The need for good differential and/or integral linearity will be
discussed in the DAC applications described in succeeding chapters.