After reading this series of articles on current sensing, you will have a solid grasp of the fundamentals of current sensing, devices that are used for current sensing, how to calculate the accuracy of a solution, and guidelines for printed circuit board (PCB) layout and troubleshooting. This article introduces the four differential amplifiers that typically are used in direct current sensing solutions: operational amplifiers (op amps), difference amplifiers (DA), instrumentation amplifiers (IA), and current shunt monitors (CSM).
The use of an op amp for current sensing is limited by input common-mode voltage. Due to the design of the input stage, input common-mode voltage of such a device is limited by the supply voltage (Voa). Additionally, the large open-loop gain of a traditional op amp requires the device to have feedback, which limits its use to single-ended input signals. Such a configuration necessitates its use to only low-side current sensing. Figure 1 depicts the use of an op amp in a low-side current sensing configuration.
Since the input common-mode voltage of the solution shown in Figure 1 is near ground, the op amp input common-mode range should include ground. It may also be desirable to select an op amp whose output is considered rail-to-rail. This yields the greatest range of load currents that can be accurately sensed. One drawback is any parasitic resistance between the shunt resistor and the ground trace adds to the shunt resistor value (see Figure 2).
The voltage developed by a parasitic resistance in this location (for example, PCB trace, solder joint) will ‘pedestal’ the shunt voltage, thereby introducing error. This parasitic resistance may vary greatly in production. For greater accuracy and consistency, a differential measurement across the shunt resistor is required.
A traditional DA is simply an op amp with a precision trimmed resistor network as shown in Figure 3. The resistors are typically trimmed during manufacturing so that R2/R1 = R4/R3.