In power supplies or battery chargers, you often need information about the current flowing in the high-side rail. Figure 1 shows a common circuit for obtaining this information.

Figure 1. |
This method for measuring high-side supply currentrequires an auxiliary transistor. |

The circuit provides a ground-referenced voltage proportional to the current flowing through the high-side sense resistor, R_{S}. The circuit needs an additional high-side supply, V_{P}. If you use a low-voltage op amp, such as an OP90, V_{P} = 1.5 V is adequate, so you can derive this supply from a series-transistor voltage drop in many applications. The op amp keeps the voltage drop in R_{S} and the conversion resistor, R_{C}, equal, so the current through R_{1} is

and the voltage across R_{1} is

Figure 2 shows a circuit that needs no auxiliary transistor. The output stage of the op amp, in a sense, replaces the transistor. The current through R_{C} is the same as that in Figure 1. Now, this current flows through the negative-supply pin of the op amp and serves as the current source for R_{1} as before. The circuit uses a low-power OP90 op amp, which draws a low supply current. The idea is that the supply current contributes a negligible offset to I_{1}. Measurements show that the positive-supply current, I_{P}, rises with current I_{1}. This situation leads to a higher current I_{1} because of the current I_{P}. Figure 3 shows the result. The dots in Figure 3 indicate the measured values of V_{1}, and the straight line shows the value if I_{P} were zero.

Figure 2. |
This circuit uses the negative-supply current of the opamp to measure the high-side supply current. |

Figure 3. |
Because of the positive-supply current, the measured values (dots)do not agree well with the theoretical values (straight line). |

Fortunately, the supply, I_{P}, is nearly proportional to the op amp's output current. So the value of R_{C} in Figure 2 is such to yield the right value at the maximum current of 2.5 A. This simple, one-point adjustment corrects the gain error. Figure 4 shows the result after this gain compensation. Now, the error between the measured V_{1} and the desired value is within 5%, which is tolerable for many applications. Measurements show that the accuracy holds for input voltages of 5 to 25 V and an additional supply voltage of 2 to 15 .

Figure 4. |
Gain compensation by adjusting R_{C} improvesthe accuracy of the measurement. |