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TL074: measure DC offset of AC voltage

Part Number: TL074


my customer is using below circuit to measure the DC offset of the 220VRMS AC voltage.

The OPA is TL074 and powered by ±12V, GND is referenced to DC bus's GND. When DC offset is 0.1V, the voltage on pin14 is 10.83V, almost the same as the +12V power rail(considering the headroom output voltage maximum should be lower than V+).

Why would that happen? Is it that the common mode input voltage is beyond recommended condition and TL074 already broken?

  • Hello Howard,

    To make things easy I'll assume the BUS+ is a nice round number, 400V. So the common mode voltage would be 200V. The signal to measure is 0.1V . The signal to noise ratio (in this case common mode) is 0.1V/200V = 0.05%.  I expect your resistor matching is not 0.01% or less. Therefore I believe the system is doomed. An isolated amplifier should be used instead.  

    If you still want to debug this circuit. Then measure "INV.DCV+" and "INV.DCV-" relative to op amp ground. Then verify that against the pin 14 output.