Current sensing revisited

Other Parts Discussed in Post: INA193, INA282, INA226

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 1: Fundamentals

Part 2: Devices

Part 3: Accuracy

Part 4: Layout and Troubleshooting Guidelines

Thanks for reading and hopefully this will help you solve your current measuring challenges!

  • Matt, the proper E2E Forum is the Current Shunt Forum:

  • If i am unsure as to which TI current sense amplifier (CSA) i should employ for my application —a household 4-bay precision charger (with LCD screen) for cylindrical batteries—, where exactly should i post my question for FREE specific help? ;D

    Here, e-mail to TI technical support or phone, or opening a thread in a forum (in which forum subsection), or or? :)

    I still watch Jerry's video and don't understand every everything mentioned in there.

    Anyway, here a more general question: Is a CSA exactly what other manufacturers, e.g. Analog Devices, call "In-amps" (Instrumentation Amplifiers)?

  • A low-side current sensing shunt can also be placed in the ground return of a power supply to monitor its load current. It is important to tie the filter capacitors, bleeder resistors, etc to the top of the shunt rather than to ground, otherwise you will be reading the capacitor ripple current, bleeder current, etc instead of only the power supply's external load current.

  • Dan, thanks for the reference. Note that further along in the video I do make the point that dedicated current

    sense amps like the INA282 and INA210 series are ideally suited for low side sensing since they provide both

    differential sensing as well as high accuracy gain setting components in a single package.

    I still do current sensing, but now I integrate it into hot swap controller and efuse devices.

    Jerry Steele