**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 IC

_{1}to measure the resulting voltage drop (

**Figure 1a**). A second method is a widely known operational amplifier current-to-voltage converter in which inverted IC

_{1}’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 IC_{1}’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 R_{1} into the output of IC_{2}, which reduces its voltage by the amount of IR_{1} relative to the input node. That voltage equals the voltage mean of the op amp’s outputs, which R_{3} and R_{4} set at the op amp’s inverting inputs. Consequently, the output of IC_{1} must rise to a voltage of IR_{2} relative to the inverting inputs and the equal-voltage noninverting input node of IC_{2}. IC_{1} sources this current, which returns through R_{2} to the circuit under test. R_{1}=R_{2}, 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.

For more detail: Measure small currents without adding resistive insertion loss