+IN

±IN

RG

INA821

5 V

OPA191

100 k

5 V

1 F

5 V

100 k

OUT

Copyright © 2017, Texas Instruments Incorporated

RG

0

20

40

60

80

100

120

10

100

1k

10k

Frequency (Hz)

C001

29

INA821

www.ti.com

SBOS893A – AUGUST 2018 – REVISED DECEMBER 2018

Product Folder Links: INA821

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Copyright © 2018, Texas Instruments Incorporated

Application Information (continued)

Figure 66. The Effect of Increasing 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 shown in Figure 67 to avoid CMRR degradation.

Figure 67. Using an Op Amp to Buffer Reference Voltages

9.1.2 Input Bias Current Return Path

The input impedance of the INA821 is extremely high (approximately 100 GΩ.) However, a path must be

provided for the input bias current of both inputs. This input bias current is typically 150 pA. High input

impedance means that this input bias current changes little with varying input voltage.

Input circuitry must provide a path for this input bias current for proper operation. Figure 68 shows various

provisions for an input bias current path. Without a bias current path, the inputs float to a potential that exceeds

the common-mode range of the INA821 and the input amplifiers saturate. If the differential source resistance is

low, the bias current return path connects to one input (as shown in the thermocouple example in Figure 68).

With a higher source impedance, using two equal resistors provides a balanced input with possible advantages

of a lower input offset voltage as a result of bias current and better high-frequency common-mode rejection.