As the voltage drop across the sense resistor (VSENSE) increases, the amount of voltage dropped across the

input filter resistors (RF) also increases. The increased voltage drop results in additional gain error. The error

caused by these resistors is calculated by the resistor divider equation shown in Equation 5.

DIFF

SENSE

DIFF

F

R

Error(%)

1

100

R

R

2 R

§

·

u

¨

¸

¨

¸

u

©

¹

(5)

where:

• RDIFF is the differential input impedance.

• RF is the added value of the series filter resistance.

The input stage of the INA186 uses a capacitive feedback amplifier topology in order to achieve high dc

precision. As a result, periodic high-frequency shunt voltage (or current) transients of significant amplitude (10

mV or greater) and duration (hundreds of nanoseconds or greater) may be amplified by the INA186, even

though the transients are greater than the device bandwidth. Use a differential input filter in these applications to

minimize disturbances at the INA186 output.

The high input impedance and low bias current of the INA186 provide flexibility in the input filter design without

impacting the accuracy of current measurement. For example, set RF = 100 Ω and CF = 22 nF to achieve

a low-pass filter corner frequency of 36.2 kHz. These filter values significantly attenuate most unwanted high-

frequency signals at the input without severely impacting the current sensing bandwidth or precision. If a lower

corner frequency is desired, increase the value of CF.

Filtering the input filters out differential noise across the sense resistor. If high-frequency, common-mode noise

is a concern, add an RC filter from the OUT pin to ground. The RC filter helps filter out both differential and

common mode noise, as well as internally generated noise from the device. The value for the resistance of the

RC filter is limited by the impedance of the load. Any current drawn by the load manifests as an external voltage

drop from the INA186 OUT pin to the load input. To select the optimal values for the output filter, use Output

Impedance vs. Frequency and see the Closed-Loop Analysis of Load-Induced Amplifier Stability Issues Using

ZOUT application report

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INA186

SBOS318B – APRIL 2019 – REVISED JULY 2021

Copyright © 2021 Texas Instruments Incorporated

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