of Microprocessors

232 MUSICAL ApPLICATIONS OF MICROPROCESSORS
I
t
2R
MSB
o
o
LSB
o
1
o
OUTPUT
o
1/4 V ref
1/2 Vref
1
3/4 V ref
Fig. 7-6. Resistor ladder DAC
as many resistors are used, the ease of matching their characteristics (remember
only the ratio accuracy is important) leads to better DAC performance
with varying temperature. In fact, all resistors could be of the same
value if 2R is actually two lR resistors in series. Another advantage is that
the load impedance of all of the switches is about the same. This eliminates
the need for scaling switch size; instead the switch resistance can simply be
subtracted from the 2R series resistor (or a large resistor placed in parallel
with 2R). Speed can be better because the node capacitances are spread out
rather than concentrated into one node as with the weighted resistor circuit.
Analysis of the effect of an error in a single resistor is considerably more
complicated, although the same resistor accuracy rule for guaranteed
monotonic performance still holds. Also, the linearity of this circuit is not
affected by load resistance or a direct short either. The equivalent output
impedance is essentially R.
Other variations of the resistance ladder are also used. The most
popular is the current-switching structure shown in Fig. 7-7. Essentially the
circuit has been turned upside down with Vre/ driving the ladder at what was
the output point and an op-amp current to voltage converter connected to
what was Vre/. Speedwise, this circuit is probably the best. The reason is that
voltage levels on the resistor network nodes do not change, since the ladder
current is simply switched between true ground and "virtual ground" at the
op-amp summing junction. Likewise, voltage levels at the switches do not
change. When voltage levels are constant, stray capacitances are not charged
so there is no RC time constant slowdown. The result is inherently high
overall speed nearly equivalent to the individual switch speed. Settling times
of less than 0.5 J.Lsec (neglecting the effect of the output amplifier) are
routine, and 20 nsec is possible in low-resolution designs.
Segmented DAC
One recently discovered circuit essentially combines the resistor string
method described earlier with the resistor ladder method to get the major
advantages of each. In order to understand how it works, refer back to Fig.