Summary of Sensing current on the high side
Summary (under 100 words): High-side DC current sensing commonly uses a sense resistor in the supply line, measuring the voltage drop (E = I × R). Ground-leg sensing is simple with an op amp but can cause ground-loop or chassis-ground issues, so high-side sensing is often preferred. Using a differential or instrumentation amplifier across the sense resistor is possible but typically problematic due to the very high common-mode rejection required, cost, and drift concerns. Magnetic sensing is an alternative for higher currents.
Parts used in the High-Side Current Sensing Project:
- Sense resistor (shunt), e.g., 5 mΩ
- Differential amplifier or instrumentation amplifier (inamp)
- Op amp (for ground-leg sensing configurations)
- Supply rail (12 V nominal in example)
- Load capable of 0–10 A current range
- Ground/chassis reference (for discussing grounding options)
- Magnetic field sensor (as an alternative for higher-current sensing)
As EDN’s Design Ideas editor, I see a range of design submissions, from good, to not so good. A recent DI I turned down for several reasons included a high-side current sense circuit with implementation problems. This got me thinking about the different ways to accomplish current sensing on a voltage rail.
At their heart, the majority of DC current sense circuits start with a resistance in a supply line (though magnetic field sensing is a good alternative, especially in higher-current scenarios). One simply measures the voltage drop across the resistor and scales it as desired to read current (E = I × R (if I didn’t include this, someone would complain)). If the sense resistor is in the ground leg, then the solution is a simple op-amp circuit. Everything stays referenced to ground, and you only have to be careful about small voltage drops in the ground layout.
But often, placing the sense resistor in a supply lead is the preferred approach. Why? Ground might not be available (e.g., a chassis-grounded automotive device), or you may not want device ground to be different than supply ground, which can lead to ground loops and other problems. So, what are the options?
The most obvious and explicit method is to throw a differential or instrumentation amplifier (inamp) across the sense resistor, but in practice this is rarely a good way. To accurately sense current, extremely high CMR (common-mode rejection) is usually required, which is both expensive and prone to drift.
How so? Let’s consider an example design: 0-10A, 12V nominal, 5mΩ sense resistor:
For more detail: Sensing current on the high side
- What is the basic principle behind DC current sensing?
Measure the voltage drop across a sense resistor and scale it using E = I × R. - Why might you avoid placing the sense resistor in the ground leg?
Because ground may not be available or you may create ground differences that lead to ground loops and other problems. - Why is high-side sensing often preferred?
It avoids altering device ground reference and problems associated with chassis-grounded systems. - Can you use a differential or instrumentation amplifier for high-side sensing?
Yes, but in practice it is rarely a good way due to the very high common-mode rejection required, expense, and drift. - What is a downside of using instrumentation amplifiers for accurate current sensing?
Instrumentation amplifiers require extremely high CMR, which is expensive and prone to drift. - What is an alternative sensing method for higher currents?
Magnetic field sensing is a good alternative for higher-current scenarios.
