Measure small currents without adding resistive insertion loss

In most cases, you measure current by converting it into a proportional voltage and then measuring the voltage. Figure 1 shows two typical methods of making the conversion. In one method, you insert a probing resistor, R_P, in series with the current path and use differential amplifier IC₁ to measure the resulting voltage drop (Figure 1a). A second method is a widely known operational amplifier current-to-voltage converter in which inverted IC₁’s output sinks the incoming current through the feedback resistor (Figure 1b).

In the first method, the same current that flows into one node flows from the second node, but a significant voltage drop occurs across probing resistor R_P. In the second method, the voltage drop is on the order of tens of microvolts to millivolts, depending on IC₁’s quality, but the measured current flows only into the sensing node with no return to the circuit. You can measure only currents flowing to ground.

The circuit in Figure 2 operates in a somewhat similar manner to the one in Figure 1b in that an op amp’s output sinks the incoming measured current. However, the other op amp’s output sources an equal outgoing current back to the circuit under test.

In Figure 2, input current I flows through R₁ into the output of IC₂, which reduces its voltage by the amount of IR₁ relative to the input node. That voltage equals the voltage mean of the op amp’s outputs, which R₃ and R₄ set at the op amp’s inverting inputs. Consequently, the output of IC₁ must rise to a voltage of IR₂ relative to the inverting inputs and the equal-voltage noninverting input node of IC₂. IC₁ sources this current, which returns through R₂ to the circuit under test. R₁=R₂, so the output current is the same as the input current. Because the op amp’s outputs maintain their inputs at equal voltages, the circuit under test has virtually no resistance.