I'm trying to make an instrumentation amplifier to measure a sine wave signal of 0 - 1Vpp (max) 50Hz that comes from a non-invasive current sensor (30A SCT-013-030). It gives 1Vpp at 30A current.

This is the circuit I made. The output goes to an MCU.

enter image description here

According to this note, my circuit with the selected values should have GAIN = 3. enter image description here

I would like to know if this circuit is okay for that kind of application.

Also, I found on the internet that in AC-coupled signals require a common-mode voltage applied to both inputs of the circuit. And that voltage should be the less of the maximum value of the signal which in my case would be 0.5V enter image description here Do I need that voltage in my case?
What does it do exactly?

Link to the schematics

  • \$\begingroup\$ @BrianDrummond I posted the link \$\endgroup\$
    – MrBit
    Commented Mar 31, 2020 at 15:13
  • \$\begingroup\$ Which microcontroller you are using? ATMEGA 16, 32 (I used these two) have differential amplifiers in it. \$\endgroup\$
    – Sadat Rafi
    Commented Mar 31, 2020 at 15:22
  • \$\begingroup\$ If precision and common mode suppression are important, you should consider a dedicated instrumentation amplifier. Those have laser-trimmed matched resistors on-chip. \$\endgroup\$
    – user107063
    Commented Oct 11, 2023 at 9:05

1 Answer 1


AC coupling would make your current reading frequency related. The transformer has to have somehow a reference potential, i.e not floating. If you have a single supply, then it should be biased to Vref/2.


simulate this circuit – Schematic created using CircuitLab

As said, best way is to have DC coupled signal with anti-alias filter. You will have a DC offset, that you can eliminate it with high sampling and HF pass digital filter (for example > 20 Hz pass). In such way you have all harmonics with true amplitude, no need to re calibrate for 50/60Hz.


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