Current sensing devices are often desired in the power supply lines of various circuits, such as battery-powered computers, battery chargers or critical circuits requiring thermal forecasting. Figure 1 shows a simple current sensing circuit that operates over a wide supply range. The circuit uses a micropower LT1494 op amp for its rail-to-rail input and output and ultralow supply current. It requires only 1.5uA supply current, which provides excellent precision for current sensing devices.
Circuit operation is straightforward. The current through the sense resistor (RSENSE) creates a voltage across it, which is then imposed across RA by the op amp. The current through RA a flows into the op amp's output and out of the negative supply pin. This acts as a current source through RB, creating VOUT. (VOUT = IL(RB/RA)RS). Rail-to-rail operation is required because the inputs and output operate very near the upper rail when IL is small. The ultra low supply current makes IS very close to zero amps with an output offset of about 1uA.
The supply current IS is virtually proportional to the output current. This yields excellent linearity, as shown in Figure 2. The circuit will operate as long as VOUT is below (VS -- 2.4V); that is to say, the output will clip at (VS -- 2.4V).
Figure 1 (left): LT1494 circuit design; Figure 2 (right): ILOAD vs VOUT