Micropower Circuit Monitors Positive Supply Current

The inexpensive circuit of Figure 1 converts the load current of a positive power supply to a ground-referenced signal voltage, without recourse to the instrumentation amplifier, extra power supply, and matched sets of resistors typical of such circuits. The output current I_O (proportional to supply current) flows through R_O to produce V_O. Because I_O is generated by a true current source, you can reference V_O to ground or to any reasonable level within the supply range. The measurement is independent of variations in the supply voltage.

Because the op amp's common-mode range includes the supply rails, it can sense small voltages near the positive rail, such as those across R_S. Feedback resistor R_F should equal 100R_S or 1000R_S. The op amp drives P-channel MOSFET Q₁, whose drain-source current produces a voltage across R_F equal to that across R_S, subject to an error of ±V_OS. As a result,

I_O = (I_LR_S)(1/R_F) and,
V_O = (I_LR_S)(R_O/R_F).

The component values shown provide a V_O range of 0 to 1V for the supply-current range 0 to 1A. You can add a trimming potentiometer to null V_OS. The remaining gain error depends on the tolerance of R_S, R_F or R_O. The op amp draws 20µA and operates with a voltage as low as 2.5V. This op amp supply is produced by the five diodes, which are biased by R_Z and the input supply voltage as shown in the table.