A common systems requirement is to monitor the current being delivered to a load. Monitoring makes sure that normal current levels are being maintained, and also provides an alert if an overcurrent condition occurs.

Fortunately, a relatively simple current monitor design can be achieved using a precision rail-to-rail input/output op amp such as the OPAx186. This device has an input common-mode voltage (V_CM) range that extends 200 mV beyond each power supply rail allowing for operation at the supply rail.

The OPAx186 is configured as a difference amplifier with a predetermined gain. The difference amplifier inputs are connected across a sense resistor through which the load current flows. The sense resistor can be connected to the high side or low side of the circuit through which the load current flows. Commonly, high-side current sensing is applied. Figure 8-2 shows an applicable OPAx186 configuration. Low-side current sensing can be applied as well if the sense resistor can be placed between the load and ground.

Use the following parameters for this design example:

• Single supply: 24 V
• Linear output voltage range: 0.3 V to 3.3 V
• Load current, I_L: 1 A to 11 A

The following design details and equations can be used to reconfigure this design for different output voltage ranges and current loads.