To measure a 440VAC voltage, I am using a 2ma/2ma transformer. In the PCB I am using 3 exactly the same circuits to measure voltage in a three phase system.

To test the circuit, I measured output of the Op-Ams (U2, U3, U4) with FLUKE 190-504SCOPEMETER oscilloscope after removing the transformers. I observe a sine wave noise of 12.5 kHz (approx.) is introduced at an amplitude of 100 mV (approx.) at the output of the first and third op-amps. The output of the second op-amp doesn't show any noise.

In the following, I am providing the circuit diagram and the oscilloscope screenshots.

The schematic and the PCB design are available at the following links.

Full project:






Oscilloscope's screenshots

Voltage measured at the Output (PIN 1) of U2

U2 output with AC coupling

U2 output with DC coupling

Voltage measured at the Output (PIN 1) of U3

U3 output with AC coupling

U3 output with DC coupling

Voltage measured at the Output (PIN 1) of U4

U4 output with AC coupling

U4 output with DC coupling

Please help me understand where does the noise originate and why is it not showing up at the output of the U3.

  • 2
    \$\begingroup\$ 12.5kHz sounds like it could be oscillation due to insufficient op-amp power bypass capacitors, or excessive output capacitance. Try soldering a leaded 0.1uF polyester cap directly across the opamp's + and - power pins. \$\endgroup\$
    – rdtsc
    Jul 20 at 16:50
  • \$\begingroup\$ @rdtsc, I have a 0.1uF capacitor already. Do you want me to connect another one? \$\endgroup\$
    – SKGadi
    Jul 20 at 17:36
  • \$\begingroup\$ @rdtsc, Regarding output capacitance, I have a 100ohm resistor in series before connecting a load of 33nF capacitor. \$\endgroup\$
    – SKGadi
    Jul 20 at 17:44


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