Circuit Measures Small Currents Referenced To High-Voltage Rails

Designs that need to measure small signals riding on high-voltage power rails suggest the use of isolation devices. However, a high-common-mode-voltage instrumentation amplifier used with a rail-to-rail input and output amplifier can recover the signal cleanly and at a much lower cost.

Consider the system where a 5- to 500-µA current is flowing in a circuit at -75 V. A 1-kΩ sense resistor will not disturb the circuit in this instance and will provide a 5- to 500-mV signal. Direct application of the sense voltage to a difference amplifier is not advised because the magnitude of the sense resistor will unbalance the input of the difference amplifier. This will compromise the common-mode rejection of the difference amplifier. A solution is shown in Figure 1.

The voltage developed across the 1-kΩ sense resistor, R_S, is applied to the noninverting input of the OPA340. With a gain of 10, the output of the OPA340 swings from 50 mV to 5 V above the -75-V rail.

A simple zener-regulated 5.1-V supply is used to power the OPA340. This minimal regulator is adequate because the load current variation from minimum to maximum is 5 µA. This design is based on a load of 1.1 mA for the op amp and 5 mA for the zener diode. The value of R_Z is then set at (75 V ­ 5 V)/(6.1 mA) or 11.5 kΩ. Since the current through the zener is relatively constant, the change in supply for the op amp will be insignificant.

The INA146 is furnished with ±15-V supplies about ground. The input stage of the INA146 has a gain of 0.1 V/V. This gain ratio, accomplished with 100-kΩ input resistors, allows common-mode voltages of ±100 V. A gain of 20 V/V is set with the 200-kΩ and 10.5-kΩ second-stage gain resistors. The output signal will now swing between 0.2 V and 10 V.

Note that while the output signal is only positive, a negative supply is required for the INA146. This supply is necessary to allow the large negative common-mode voltage.

Variations in the -75-V rail will cause the output of the OPA340 to change but will be rejected from the INA146 output by its common-mode rejection of 80 dB. This translates to 100 µV of output change per volt of change in the -75-V rail.

For the situation where the signal is sensed from a high positive supply, a similar circuit as shown in Figure 2 can be used.