AN833

2

Rev. 1.1

Figure 2. Measured V_iload vs. Input Current

Amplifier input offset voltages need to be considered carefully in this circuit. The TS1001 op amp is specified with

±3 mV maximum input offset voltage at 25 °C, which corresponds to ±3 mA of error. Figure 2 shows an offset of

approximately 2.5 mA. Another consideration is that Q1 exhibits drain-source leakage current of a few 10s of

nanoamps at 25 °C and can approach 1µA over the commercial temperature range. Since the current through Q1

is effectively controlled by the TS1001’s loop, any drain-source leakage from Q1 appears as a current floor and

generates a corresponding minimum voltage output across R3 below which current cannot be measured until the

op amp “takes over” at higher measured currents. Therefore, normal methods of removing the current sense

amplifier input offset voltage by subtracting the zero-load voltage at V_iin will not work, since the offset voltage due

to the op amp’s Vos and the current floor from Q1’s drain-source leakage cannot be separated.

Table 1. Current Sense Amplifier Circuit Supply Current vs. Load Current

Load Current

Current Used by the Current Sense Amplifier

0

860 nA

50 mA

48.1 µA