Figure 1. This current-sensing circuit monitors a negative power supply and provides a positive output voltage proportional to the load current.
Voltages at the inverting and noninverting terminals of the op amp (IC1A) are forced to be equal by an active-feedback current mirror. VR1 = VSENSE and therefore:
Three alternatives are now possible. You can convert the output current (IR1) to voltage by connecting resistor RO to ground, to VCC, or to an inverting amplifier. Connecting RO to ground (GND) eliminates the need for a positive supply. In that case, the output voltage is negative and proportional to load current:
You can connect RO to VCC for applications that require a positive output voltage, but the output will be referenced to VCC:
To reference the positive output voltage to ground, you must use an inverting amplifier (IC1B), as shown in Figure 1:
Note that RO does not affect output voltage for the inverting-amplifier, but this resistor is usually needed for stability. RG can be optional, but it also provides stability by isolating the op amp from the capacitive load of the MOSFET gate. Finally, RC compensates for the op amp's input bias current.
Figure 2 shows measurement error vs. load current for the Figure 1 circuit. To ensure accurate current measurements, the resistors (except for RG and RC) should have a tolerance of 1% or better. RSENSE must be rated to dissipate the power associated with high load currents.
Figure 2. Error for the current sensor of Figure 1 is less than 2% at full scale, but the op amp's inherent input-offset voltage reduces the accuracy at lower levels of current.
A similar article appeared in the September, 2005 issue of Power Electronics Technology.