This imbalance results in a degraded common-mode rejection ratio (CMRR). Figure 9-2 shows how the

common-mode rejection ratio degrades depending on the source resistance on the reference pin. For best

performance, keep the dc CMRR greater than 100 dB by keeping the source impedance to the REF pin

(represented as R1) to less than 0.1 Ω.

Figure 9-2. Effect of Parasitic Resistance at the Reference Pin

Voltage-reference devices are an excellent option for providing a low-impedance voltage source for the

reference pin. However, if a resistor voltage divider generates a reference voltage, the divider must be buffered

by an op amp (as Figure 9-3 shows) to avoid CMRR degradation.

+

±

100 k

100 k

5 V

5 V

í5 V

1 …F

OUT

INA849

REF

+IN

íIN

5 V

OPA320

Figure 9-3. Using an Op Amp to Buffer Reference Voltages

INA849

SBOS945B – NOVEMBER 2020 – REVISED APRIL 2021

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