Current monitoring is a critical function in a wide range of applications. While several current sensing methods exist, a shunt resistor provides a highly accurate, and robust approach. Designers need to choose between high-side and low-side current measurements. High-side measurement is often preferred where accuracy and load protection are critical.

High-side current sense circuits require special consideration of the input and output common-mode limitations of the amplifier. The biggest challenge in high-side current measurements is making sure that the amplifier can support and mitigate the errors that are inherent to high common-mode voltage. Designers can attenuate the common-mode voltage by using a difference amplifier, but significant gain and input impedance trade-offs exist. High input impedance and large gain is especially critical when measuring small currents. In such cases, an instrumentation amplifier provides the required input impedance, and gain.

The trade-off is that instrumentation amplifiers have a complex input voltage range that depends on the common-mode voltage, differential voltage, gain, and reference voltage. The input stage alone is bound by the input common-mode voltage limitation and the maximum power supply voltage range of the operational amplifier. Fortunately, the OPAx591 offers a high operating power supply voltage that enables designers to build a very high common-mode, high impedance, high gain, high common-mode rejection instrumentation amplifier. This instrumentation amplifier can be used to measure small differential signals in the presence of very high common-mode voltage.