Current-sense amplifier precisely measures low side
Several integrated high-side current-sense amplifiers, such as the MAX4172 (Maxim Integrated Products, www.maxim-ic.com), make it easy to measure the current from a positive power supply. With a couple of resistors, these devices provide a ground-referenced voltage output that is proportional to the delivered current (Figure 1a).
Unfortunately, no equivalent devices implement a true low-side current sense for negative supplies. You can use the high-side devices to measure current from negative supplies, but this approach has drawbacks. The sense resistor must be in the load's ground lead, which effectively floats the load off ground by the sense resistor voltage, and the output voltage, VOUT, is referred to the negative supply and not to ground (Figure 1b). Both of these characteristics are undesirable.
The circuit in Figure 2 implements a true low-side, precision current-sense amplifier that provides a ground-referenced voltage proportional to the negative supply current and that does not float the load off ground. This circuit is similar to the circuit in the MAX4172 but is reconfigured for a negative supply. When current flows through the load, a proportional voltage, VSENSE, develops across the current-sense resistor RSENSE: VSENSE=ILOAD*RSENSE. To stay balanced, the op amp raises its output until VSENSE also appears across R1. The current through R1 equals VSENSE/R1, which is identical to the current through R2, which scales VOUT. The equation for VOUT is then as follows:
To minimize the effects of the op amp's offset voltage, the voltage developed across the sense resistor should be larger than this offset value. The circuit in Figure 2 develops a 0 to –5V output for load currents of 0 to 3A and is accurate to about 1%.
You must adhere to the following component restrictions: The op amp needs to have an input common-mode range that includes the negative rail. The op-amp output needs to be a rail-to-rail type. Q1 should have a high beta at low currents to minimize errors due to base current. Keep R1 small so that op-amp bias currents are insignificant. For greater accuracy, you can replace Q1 with an enhancement-mode n-channel MOSFET to eliminate base-current errors; however, operation as low as 3V is then unlikely. You can also replace the op amp with a lower offset device at the expense of high-voltage operation to –44V. (DI #2410)