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 tolerance and common-mode noise. The second approach uses V+ as the reference instead of ground, offering a simpler solution in certain applications where grounding may not be ideal. The third method, commonly found in integrated circuit (IC) solutions, leverages transistors in combination with an op-amp to provide a ground-referenced current measurement, making it more robust and suitable for high-side sensing.
As an editor for the EDN "Design Example" column, I regularly review a wide range of design submissions. Some are well thought out, while others have significant flaws. Recently, I came across a high-end current detection circuit that had several issues, prompting me to revisit different approaches to current sensing on a voltage rail.
Most DC current detection circuits begin by placing a sense resistor in the power line. Although magnetic field-based methods like current transformers are also viable—especially for high-current applications—the most common approach is to measure the voltage drop across a low-value resistor. This voltage is then amplified or processed to determine the current value using Ohm’s Law (E = I × R). When the sense resistor is placed in the ground return path, a simple op-amp circuit can be used, and everything remains grounded. However, care must be taken to minimize the voltage drop in the grounding layout to avoid errors.
In many cases, however, it's preferable to place the sense resistor in the power line rather than the ground path. This is especially true when grounding through the chassis is involved, or when avoiding ground loops is essential. So, what's the best way to measure current in such scenarios?
The most obvious solution is to use a differential or instrumentation amplifier across the sense resistor. But this isn't always the best choice, as it requires a very high common-mode rejection ratio (CMRR), which can be expensive and prone to drift over time.
Let’s take a closer look at a practical example: 0–10A, 12V nominal, with a 5mΩ sense resistor. In this case, even a small offset voltage can lead to significant current errors. For instance, a 1V supply offset and an 80dB CMRR could result in a current drift of around 20mA, which is clearly problematic for precision applications.
Although a three-op-amp instrumentation amplifier is less sensitive to resistor matching than a single op-amp differential amplifier, there's often a better alternative. One design example I reviewed used a single op-amp differential amplifier with discrete resistors. While this works if the supply is stable, it’s generally not recommended due to its susceptibility to variations and lack of flexibility.
Another innovative approach involves using V+ as the reference instead of ground. This method is conceptually similar to measuring a negative voltage source, and it can be effective if properly level-shifted. This technique simplifies the circuit and avoids some of the challenges associated with high-side sensing.
The third and most widely used method today is found in IC solutions, where transistors and op-amps work together to create a ground-referenced current measurement. For example, STMicroelectronics’ TSC103 uses a BJT, while Linear Technology’s LTC6102 employs a MOSFET. These ICs simplify the design and improve accuracy, but it's also possible to build similar circuits from discrete components.
In summary, choosing the right current-sensing method depends on the application’s requirements, including cost, accuracy, and ease of implementation. Whether you opt for a discrete op-amp setup, a clever reference scheme, or an integrated solution, understanding the trade-offs is key to designing a reliable and accurate current detection system.
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