After reading the series of articles on current detection, you will have a solid understanding of the basics of current detection, equipment used for current detection, how to calculate the accuracy of solutions, and guidelines for printed circuit board (PCB) layout. And troubleshooting. This article introduces four differential amplifiers commonly used in DC sensing solutions: operational amplifier (op amp), differential amplifier (DA), instrumentation amplifier (IA), and current shunt monitor (CSM).

Operational amplifier

The input common mode voltage limits the use of operational amplifiers in current detection. Due to the design of the input stage, the input common mode voltage of this device is limited by the power supply voltage (Voa). In addition, the large open-loop gain of traditional operational amplifiers requires the device to have feedback, which limits its use for single ended input signals. This configuration makes it only suitable for low-end current sensing. Figure 1 illustrates the use of an operational amplifier in a low-end current detection configuration.

Due to the input common mode voltage of the solution shown in Figure 1 being close to ground, the input common mode range of the operational amplifier should include ground. It may also be necessary to choose an operational amplifier that is paired with a current sensor, whose output is considered rail to rail. This generates the maximum range of load current that can be accurately detected. One disadvantage is that any parasitic resistance between the shunt resistor and the ground wire will increase the value of the shunt resistor (see Figure 2).

The voltage generated by parasitic resistance at that location (e.g. PCB routing, solder joints) will "shunt" the shunt voltage, thereby introducing errors. This parasitic resistance may undergo significant changes during production. In order to achieve higher accuracy and consistency, differential measurements need to be taken at both ends of the shunt resistor.

Differential amplifier

The traditional DA is only an operational amplifier with a precisely adjusted resistor network, as shown in Figure 3. The resistance of current sensor is usually adjusted during manufacturing to make R2/R1=R4/R3. Therefore, the differential gain (Adm) of the device is R2/R1. Add the reference voltage (Vref) to the output voltage (Vo).

Regarding current detection, due to the resistive voltage divider at the input end, DA can have an input common mode voltage that exceeds the power supply voltage, as shown in Figure 4. This allows the use of DA for high-end current detection. However, due to its limited common mode and differential mode input impedance, DA can impose a burden on the system bus voltage. The load draws current from the system bus voltage, which introduces uncertainty in the measurement. In order to reduce measurement errors caused by these input impedances, they should be significantly greater than the system load impedance.

Due to the common mode voltage of the low-end current sensing solution being close to 0V, DA can be used, as shown in Figure 5. This can minimize the impact of common mode input impedance, but differential mode input impedance is still a factor. The use of DA in low side measurements can eliminate the problem caused by parasitic resistance grounding, which is connected in series with the parallel resistance discussed in the operational amplifier section. Finally, DA has a fixed differential gain because the resistor network must be adjusted to maintain a good common mode rejection ratio. Some DAs have on-chip in-phase amplifiers whose gain can be adjusted. If other gains are required, it is recommended to choose such devices or increase the gain of the DA output through an external operational amplifier circuit.

In short, DA can be used for high-end or low-end sensing. When used for high-end detection, errors may be caused by limited common mode and differential mode input impedances. When used for low edge detection, DA can eliminate any issues caused by parasitic ground resistance in series with parallel resistors.