Datasheet

Data Sheet AD8421
Rev. 0 | Page 25 of 28
APPLICATIONS INFORMATION
DIFFERENTIAL OUTPUT CONFIGURATION
Although the dc performance and resistor matching of the op amp
affect the dc common-mode output accuracy, such errors are
likely to be rejected by the next device in the signal chain and,
therefore, typically have little effect on overall system accuracy.
Figure 70 shows an example of how to configure the AD8421 for
differential output.
+IN
–IN
REF
AD8421
V
BIAS
+
–
OP AMP
+OUT
–OUT
10123-066
12pF
10kΩ
10kΩ
Because this circuit is susceptible to instability, a capacitor is
included to limit the effective op amp bandwidth. This capacitor
can be omitted if the amplifier pairing is stable.
The open-loop gain and phase of any amplifier may vary with
process variation and temperature. Additional phase lag can be
introduced by resistive or capacitive loading. To guarantee
stability, the value of the capacitor in Figure 70 should be
determined with a sample of circuits by evaluating the small signal
pulse response of the circuit with load at the extremes of the
output dynamic range.
Figure 70. Differential Output Configuration with Op Amp
The ambient temperature should also be varied over the expected
range to evaluate its effect on stability. The voltage at +OUT may
still have some overshoot after the circuit is tuned because the
AD8421 output amplifier responds faster than the op amp. A 12 pF
capacitor is a good starting point.
The differential output voltage is set by the following equation:
V
DIFF_OUT
= V
+OUT
− V
−OUT
= Gain × (V
+IN
− V
−IN
)
The common-mode output is set by the following equation:
V
CM_OUT
= (V
+OUT
+ V
−OUT
)/2 = V
BIAS
For best large signal ac performance, use an op amp with a high
slew rate to match the AD8421 performance of 35 V/µs. High
bandwidth is not essential because the system bandwidth is limited
by the RC feedback. Some good choices for op amps are the
AD8610, ADA4627-1, AD8510, and the ADA4898-1.
The advantage of this circuit is that the dc differential accuracy
depends on the AD8421, not on the op amp or the resistors. In
addition, this circuit takes advantage of the precise control that the
AD8421 has of its output voltage relative to the reference voltage.