AD8421ARMZ-R7 datasheet(26/29 Pages) AD

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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

VBIAS

–

OP AMP

+OUT

–OUT

12pF

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:

VDIFF_OUT = V+OUT − V−OUT = Gain × (V+IN − V−IN)

The common-mode output is set by the following equation:

VCM_OUT = (V+OUT + V−OUT)/2 = VBIAS

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.

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