An important aspect of the INA226 is that the device does not necessarily measure current or power. The device measures both the differential voltage applied between the IN+ and IN- input pins and the voltage applied to the VBUS pin. For the device to report both current and power values, the user must program the resolution of the Current Register (04h) and the value of the shunt resistor present in the application to develop the differential voltage applied between the input pins. The Power Register (03h) is internally set to be 25 times the programmed Current_LSB. Both the Current_LSB and shunt resistor value are used in the calculation of the Calibration Register value the device uses to calculate the corresponding current and power values based on the measured shunt and bus voltages.

The Calibration Register is calculated based on Equation 1. This equation includes the term Current_LSB, which is the programmed value for the LSB for the Current Register (04h). The user uses this value to convert the value in the Current Register (04h) to the actual current in amperes. The highest resolution for the Current Register (04h) can be obtained by using the smallest allowable Current_LSB based on the maximum expected current as shown in Equation 2. While this value yields the highest resolution, selecting a value for the Current_LSB to the nearest round number above this value is common to simplify the conversion of the Current Register (04h) and Power Register (03h) to amperes and watts respectively. The R_SHUNT term is the value of the external shunt used to develop the differential voltage across the input pins. 

where

• 0.00512 is an internal fixed value used to verify scaling is maintained properly

After programming the Calibration Register, the Current Register (04h) and Power Register (03h) update accordingly based on the corresponding shunt voltage and bus voltage measurements. Until the Calibration Register is programmed, the Current Register (04h) and Power Register (03h) remain at zero.