SBOS386A May 2019 – May 2020 INA186-Q1
When performing accurate current measurements in noisy environments, the current-sensing signal is often filtered. The INA186-Q1 features low input bias currents. Therefore, adding a differential mode filter to the input without sacrificing the current-sense accuracy is possible. Filtering at the input is advantageous because this action attenuates differential noise before the signal is amplified. Figure 31 provides an example of how to use a filter on the input pins of the device.
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.
The input stage of the INA186-Q1 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-Q1, even though the transients are greater than the device bandwidth. Use a differential input filter in these applications to minimize disturbances at the INA186-Q1 output.
The high input impedance and low bias current of the INA186-Q1 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-Q1 OUT pin to the load input. To select the optimal values for the output filter, use Figure 24 and see the Closed-Loop Analysis of Load-Induced Amplifier Stability Issues Using ZOUT application report