The differential amplifier used for this design should have the following features:

1. Single-supply (3 V),
2. Reference voltage input,
3. Low initial input offset voltage (V_OS),
4. Low-drift,
5. Fixed gain, and
6. Low-side sensing (input common-mode range below ground).

For this design, a current-shunt monitor (INA213) is used. The INA21x family topology is shown in Figure 10-3. The INA213B specifications can be found in the INA213 product data sheet.

Figure 10-3 INA21x Current-Shunt Monitor Topology

The INA213B is an excellent choice for this application because all the required features are included. In general, instrumentation amplifiers (INAs) do not have the input common-mode swing to ground that is essential for this application. In addition, INAs require external resistors to set their gain, which is not desirable for low-drift applications. Difference amplifiers typically have larger input bias currents, which reduce solution accuracy at small load currents. Difference amplifiers typically have a gain of 1 V/V. When the gain is adjustable, these amplifiers use external resistors that are not conducive to low-drift applications.