AD711
OPTIMIZING SETTLING TIME
Most bipolar high-speed D/A converters have current outputs;
therefore, for most applications, an external op amp is required
for current-to-voltage conversion. The settling time of the con-
verter/op amp combination depends on the settling time of the
DAC and output amplifier. A good approximation is:
t
S
Total
=
(t
S
DAC
)
2
+
(t
S
AMP
)
2
In addition to a significant improvement in settling time, the
low offset voltage, low offset voltage drift, and high open-loop
gain of the AD711 family assures 12-bit accuracy over the full
operating temperature range.
The excellent high-speed performance of the AD711 is shown in
the oscilloscope photos of Figure 25. Measurements were taken
using a low input capacitance amplifier connected directly to the
summing junction of the AD711 – both photos show the worst
case situation: a full-scale input transition. The DAC’s 4 kΩ
[10 kΩ 8 kΩ = 4.4 kΩ] output impedance together with a 10 kΩ
feedback resistor produce an op amp noise gain of 3.25. The
current output from the DAC produces a 10 V step at the op
amp output (0 to –10 V Figure 25a, –10 V to 0 V Figure 25b.)
Therefore, with an ideal op amp, settling to
±
1/2 LSB (± 0.01%)
requires that 375
µV
or less appears at the summing junction.
This means that the error between the input and output (that
voltage which appears at the AD711 summing junction) must be
less than 375
µV.
As shown in Figure 25, the total settling time
for the AD711/AD565 combination is 1.2 microseconds.
The settling time of an op amp DAC buffer will vary with the
noise gain of the circuit, the DAC output capacitance, and with
the amount of external compensation capacitance across the
DAC output scaling resistor.
Settling time for a bipolar DAC is typically 100 to 500 ns. Previ-
ously, conventional op amps have required much longer settling
times than have typical state-of-the-art DACs; therefore, the
amplifier settling time has been the major limitation to a
high-speed voltage-output D-to-A function. The introduction of
the AD711/712 family of op amps with their 1
µs
(to
±
0.01% of
final value) settling time now permits the full high-speed capa-
bilities of most modern DACs to be realized.
Figure 24.
±
10 V Voltage Output Bipolar DAC
a. (Full-Scale Negative Transition)
b. (Full-Scale Positive Transition)
Figure 25. Settling Characteristics for AD711 with AD565A
REV. A
–7–
AD [ ANALOG DEVICES ]
