SBOS893C August 2018 – July 2019 INA821
The output voltage of the INA821 is developed with respect to the voltage on the reference pin (REF.) Often, in dual-supply operation, REF (pin 6) connects to the low-impedance system ground. In single-supply operation, offsetting the output signal to a precise midsupply level is useful (for example, 2.5 V in a 5-V supply environment). To accomplish this level shift, a voltage source must be connected to the REF pin to level-shift the output so that the INA821 drives a single-supply analog-to-digital converter (ADC).
The voltage source applied to the reference pin must have a low output impedance. As shown in Figure 65, any resistance at the reference pin ( RREF in Figure 65) is in series with one of the internal 10-kΩ resistors.
The parasitic resistance at the reference pin (RREF) creates an imbalance in the four resistors of the internal difference amplifier that results in a degraded common-mode rejection ratio (CMRR). Figure 66 shows the degradation in CMRR of the INA821 as a result of the increased resistance at the reference pin. For the best performance, keep the source impedance to the REF pin (RREF) less than 5 Ω.
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 67 shows, to avoid CMRR degradation.