Since the common-mode voltage of a low-side current sensing solution is near 0V, a DA can be used as shown in Figure 5. In order to reduce the measurement error due to these input impedances, they should be significantly larger than the system load impedance.
This load draws current from the system bus voltage, which introduces uncertainty in the measurement. However, a DA places a load on the system bus voltage due to its finite common-mode and differential-mode input impedances. This allows one to use a DA for high-side current sensing. With respect to current sensing, a DA can have input common-mode voltages outside of the supply voltages due to the resistive divider at the inputs, as shown in Figure 4. The reference voltage (Vref) is added to the output voltage (Vo). The differential gain (Adm) of the device is therefore R2/R1. The resistors are typically trimmed during manufacturing so that R2/R1 = R4/R3. For greater accuracy and consistency, a differential measurement across the shunt resistor is required.Ī traditional DA is simply an op amp with a precision trimmed resistor network as shown in Figure 3. This parasitic resistance may vary greatly in production. The voltage developed by a parasitic resistance in this location (for example, PCB trace, solder joint) will ‘pedestal’ the shunt voltage, thereby introducing error. One drawback is any parasitic resistance between the shunt resistor and the ground trace adds to the shunt resistor value (see Figure 2). This yields the greatest range of load currents that can be accurately sensed.
It may also be desirable to select an op amp whose output is considered rail-to-rail. 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. Figure 1 depicts the use of an op amp in a low-side current sensing configuration. Such a configuration necessitates its use to only low-side current sensing.
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. Due to the design of the input stage, input common-mode voltage of such a device is limited by the supply voltage (Voa). The use of an op amp for current sensing is limited by input common-mode voltage. 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).
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.