Although the INAx181-Q1 do not have a shutdown pin, the low power consumption of these devices allows the output of a logic gate or transistor switch to power the INAx181-Q1. This gate or switch turns on and off the INAx181-Q1 power-supply quiescent current.
However, in current shunt monitoring applications, there is also a concern for how much current is drained from the shunt circuit in shutdown conditions. Evaluating this current drain involves considering the simplified schematic of the INAx181-Q1 in shutdown mode, as shown in Figure 46.
There is typically more than 500 kΩ of impedance (from the combination of 500-kΩ feedback and
input gain set resistors) from each input of the INAx181-Q1 to the OUT pin and to the REF pin. The amount of current flowing through these pins depends on the voltage at the connection. For example, if the REF pin is grounded, the calculation of the effect of the 500 kΩ impedance from the shunt to ground is straightforward. However, if the reference is powered while the INAx181-Q1 is in shutdown mode, instead of assuming 500 kΩ to ground, assume 500 kΩ to the reference voltage.
Regarding the 500-kΩ path to the output pin, the output stage of a disabled INAx181-Q1 does constitute a good path to ground. Consequently, this current is directly proportional to a shunt common-mode voltage present across a 500-kΩ resistor.
As a final note, as long as the shunt common-mode voltage is greater than VS when the device is powered up, there is an additional and well-matched 55-µA typical current that flows in each of the inputs. If less than VS, the common-mode input currents are negligible, and the only current effects are the result of the 500-kΩ resistors.