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, the amount of current drained from the shunt circuit in shutdown conditions is also a concern. Evaluating this current drain involves considering the simplified schematic of the INAx181-Q1 in shutdown mode, as shown in Figure 7-8.

Figure 7-8 Basic Circuit to Shut Down the INA181-Q1 With a Grounded Reference

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, the input current is determined by the 500-kΩ impedance and the voltage difference between the positive input and the voltage applied 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 long as the shunt common-mode voltage is greater than V_S 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 V_S, the common-mode input currents are negligible, and the only current effects are the result of the 500-kΩ resistors.