Design Ideas: March 16, 1995
Measuring dc would be simple if you could always reference your measurement signals to the positive rail. Unfortunately, most current-metering applications require a ground-referenced output. Typically, the signal that you want to measure consists of a small differential signal that rides on a large common-mode voltage.
The circuit in Fig 1 can measure dc in a 3 to 70V supply line without requiring a large voltage drop across the sensing resistor, R1. For the values in Fig 1, each 10A of supply current produces a 10V output (IC1's output). R1 would dissipate only 1W from a 50V, 10A supply because of its low 10-mohm value.
Matched transistor pair Q1A and Q1B provide common-mode rejection and convert the current-sense voltage developed across R1 to a differential current. Differential current-to-voltage converter IC1 provides a single-ended output proportional to the load current.
Q2 establishes the circuit's quiescent current. Q2 and D1 form a temperature-compensated current source, developing ~200 µA. Diodes Q1C and Q1D are current mirrors for Q1A and Q1B, stabilizing Q1A's and Q1B's quiescent collector currents.
Setting Q2's emitter at ground produces a virtual ground at both op-amp inputs via the current-mirror action, establishing the output-reference (zero-current) voltage. R10 sets the circuit's gain, and potentiometer R11 provides an offset trim.
For lowest drift and minimum sensitivity to supply-voltage changes, you should thermally couple Q1A and Q1B closely. For a quad-transistor package, use the devices at the same end of the package as Q1A and Q1B. Trimming Q1A and Q1B for less than ~4 mV of initial offset ensures <1% full-scale change in offset voltage over a 5 to 50V supply range and negligible temperature-induced drift.
Wire R1 carefully to eliminate parasitic-wiring resistance. Parallel a package of eight resistors to achieve the 5-kohm resistors connecting the transistors to sense resistor R1 to minimize parts count and offset trimming. (DI #1670)