Back a couple of years ago, TI systems engineer, Jerry Steele, put out a great video called “Current Sensing: Low Side, High Side, and Zero Drift” on the different methods for current sensing. In the video, he discusses low-side current measurement – when the shunt resistor is between the load (or the supply) and ground. He explains that the positives of low-side sensing are the common-mode voltage being essentially 0V and that it is a very simple and straight forward method to measure current. The biggest drawback being that the load (or supply) is isolated from system ground by the shunt resistor (see Figure 1). This prevents the detection of load shorts to ground, which could lead to system damage. It also means that it is a single-ended measurement – more on this in a second.
Figure 1: Example of low-side current measurement
Jerry’s video also introduces high-side measurements where the shunt resistor is between the supply and the load (see Figure 2). The pros of this method include easily detecting opens or shorts, as the load is connected directly to system ground, and allows for directly monitoring the current from the source itself. The potential issues include the fact that the common-mode voltage of the amplifier is now essentially the same as the supply voltage, which in some systems is extremely high. This requires input matching on the device to keep the common-mode rejection error low. Also, since this is now a differential amplifier circuit, the circuitry is more complex than the single-ended measurement that low-side measuring can use.
Figure 2: Example of high-side current measurement
Since the video’s release, TI has introduced additional high-performing devices that are ideal for high-side sensing applications. Specifically, the INA282 family supports a common-mode range of -16V to +80V with an offset of 70μV. This is over a 95% reduction over the INA193 that Jerry references. Now think about the zero drift discussion as it relates to the accuracy of the measurement and the value of the shunt resistor (full-scale shunt voltage drop) required for meeting your accuracy goals. You can see how much more easily any device in the INA282 family enables you to meet your goals.
If you don’t need the high common-mode voltage range, then in addition to the analog output INA210 family, check out the INA226 – it’s great for either low-side or high-side applications. It has industry leading offset voltage of 10μV. Combine that with the gain error of 0.1%, enabled by the input architecture and the digital output, and you can see that it provides industry leading accuracy performance regardless of the full-scale differential shunt voltage drop as shown in Figure 3.
Figure 3: Performance measurement of the INA226 bidirectional current/power monitor
If you want to read more on current measurement techniques, I highly recommend a four part series put together by our apps team and featured in EE Times:
Part 4: Layout and Troubleshooting Guidelines
Thanks for reading and hopefully this will help you solve your current measuring challenges!
