This article explores various methods for current sensing on a voltage rail, using a specific example to highlight the advantages and disadvantages of each approach. The first method involves a single op-amp differential amplifier with discrete resistors, which is straightforward but can be sensitive to resistor matching and common-mode rejection. The second approach uses V+ as the reference instead of ground, offering a more flexible solution in certain applications. The third method, commonly found in integrated circuit (IC) designs, employs transistors in conjunction with an op-amp to provide a ground-referenced current measurement.
As an editor of the EDN "Design Example" column, I frequently encounter a wide range of design submissions—some well thought out, others less so. Recently, I came across a high-end current detection circuit that was ultimately abandoned due to several issues. This experience prompted me to revisit different techniques for implementing current sensing on a voltage rail.
Most DC current detection circuits begin by placing a resistor in the power supply line. While magnetic field induction is a viable alternative, especially for high-current applications, it's often simpler to measure the voltage drop across a sense resistor. By adjusting the resistance value, you can easily calculate the current using Ohm’s Law (E = I × R). When the sense resistor is placed in the ground path, a basic op-amp circuit can be used to measure the voltage drop. Everything remains grounded, but care must be taken to minimize the voltage drop in the grounding layout.
However, the preferred method is usually to place the sense resistor in the power line. Why? Because grounding may not always be practical—such as in automotive electronics where grounding through the chassis is common—or because you want to avoid potential ground loops and other noise-related issues. So what’s the alternative?
The most obvious solution is to use a differential or instrumentation amplifier (in-amp) across the sense resistor. However, this isn't always ideal. High-precision current sensing typically requires a very high common-mode rejection ratio (CMR), which can be expensive and prone to drift over time.
Let’s look at a practical example: 0–10A, 12V nominal, with a 5mΩ sense resistor. In such a case, even small variations in the amplifier’s offset can lead to significant current errors. For instance, a 1V supply voltage offset with an 80dB CMRR could result in a current drift of about 20mA. Multiply that by the full 12V range, and you get up to 240mA of error.
While a three-op-amp instrumentation amplifier is less sensitive to resistor mismatch than a single-op-amp differential amplifier, there’s often a better way.
One of the design examples I reviewed used a single op-amp differential amplifier with discrete resistors. Interestingly, a potentiometer was used, but it turned out to be for gain adjustment rather than CMRR optimization. If the supply voltage is stable, this method can work—but it’s not the most robust solution.
Another advanced technique involves thinking outside the box. Instead of using ground as the reference, some designs use V+ as the reference point. This is similar to how negative voltage sources are handled in low-side sensing, and it can be an effective solution if properly level-shifted.
In addition to these approaches, many IC manufacturers now offer built-in current-sensing solutions. These often use transistors and op-amps together to create a ground-referenced current measurement. For example, STMicroelectronics’ TSC103 uses a BJT, while Linear Technology’s LTC6102 uses a MOSFET. These ICs simplify the design process and improve reliability.
Despite the availability of these ICs, it's still possible to implement similar circuits using discrete components. Understanding the trade-offs between different methods helps engineers choose the best approach for their specific application.
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