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I need to measure relative small currents (10 mA to 500 mA) from the high side at mains voltage (230 VAC RMS) levels. I also need a high resolution and preferably high accuracy for the measurement. The method chosen is to use a current shunt resistor and pair it with a current sense amplifier. CSA then converts the differential voltage over the current shunt to single-ended output voltage that I can measure with a microcontroller.

My question is; if I select for example an INA4180 as my CSA measuring the current shunt resistor, am I violating its specs by exposing it to line voltage levels? INA4180 is specified as having maximum common mode voltage of 26 volts but is this just the spec for voltage over the current shunt? I think I'm safe, but can you verify?

Other parts in the circuits are the microcontroller and some sort of isolation barrier over which the MCU communicates measurement values digitally. If all this "mains side" stuff is fed from an isolated DC-DC converter with no connection to mains phase or neutral (or earth), is this a viable scheme? I think the measuring system would then be floating without any galvanic connection to mains and I'm free to measure just the voltage differential generated by the current shunt resistor. I don't have any schematics drawn yet.

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  • \$\begingroup\$ Is not INA4180 used only for DC "positive" current? \$\endgroup\$
    – Antonio51
    Oct 26, 2022 at 14:47

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Yes you would be exceeding the rating of that part. Common mode is the portion of the voltage that is the same between the low side and high side of the shunt. For a low resistance shunt this voltage is approximately the supply voltage.

I think a current transformer would be a better solution for you. Very simple, and it's got isolation built in.

This seems like it might be a good reference for you:

https://wiki.analog.com/university/courses/tutorials/alm-awg-ac-mains-tests

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  • \$\begingroup\$ Thanks for the explanation and link. However, would you mind explaining about common mode a bit more? I'm confused regarding that. I think I need to research a bit more. The main reason I'm confused is that if the CSA's supply has no connection to mains neutral, it should not matter what the absolute voltage is. I'm thinking a bird sitting on a multikilovolt transmission line without any harm... \$\endgroup\$
    – entropiae
    Oct 26, 2022 at 18:36
  • \$\begingroup\$ I've been going through different reference designs for single phase energy meters and many of them connect current shunts to ADCs via simple series resistors. These ADCs allow at most +/-1 V over their VDD or VREF, so how does that work? So I think Drew's answer is not in fact correct. Or then there is something I don't understand? But I'm still inclined to think that my bird on a power line analogy is correct. If there is no reference between ADC (or CSA) ground to line neutral, where does the too high common mode voltage come from? See: ti.com/tool/TIDA-010036 \$\endgroup\$
    – entropiae
    Oct 27, 2022 at 14:26
  • \$\begingroup\$ It is technically possible to do what you are describing (floating the current meter), but it is more difficult than it sounds. Isolated power supplies only provide DC isolation. In practice you will often have issues with capacitive coupling. Remember that once you float the current meter, every piece of metal around the shunt now effectively has 230 VAC on it, that includes within the power supply. So you'll end up with coupling all over the place. \$\endgroup\$
    – Drew
    Oct 27, 2022 at 17:12

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