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OPA547: OPA 547 offset error

rudolf doelling
Prodigy 20 points
Part Number: OPA547
Other Parts Discussed in Thread: OPA551,

Hi,

I designed a zero resistance ammeter using OPA 551. Supply is +/- 15 V. The output offset was within 1 mV, esily trimmed off by feeding a small voltage to the non-inv input.

Now I replaced the OPA 551 with OPA 547 to get more current capability.

I expected an offset of somewhat below 2 mV. Indeed, on 20 identical boards, we found an offset of approx. - 7 mV (same on all boards) at the output, when pin 1 es set to GND. We had to feed 7 mV to pin 1 in order to trim off the output offset.

Is that the expected behaviour? 

Best regards

Rudolf Doelling 

  • 0
    Guru 140200 points

    Hi Rudolf,

    difficult to say without a schematic.

    The OPA551 has an input bias current of only 20pA while the OPA547 shows 100nA. This much higher input bias current can cause a much higher voltage drop across the feedback resistors of OPAmp in your circuit adding a higher unwanted offset voltage.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru* 93105 points

    Hi Rudolf,

    Indeed, if high value resistances are being used in the OPA547 circuit the much higher input bias currents that Kai mentions can affect the output referred voltage offset obtained. Can you share your circuit diagram?

    Regards, Thomas

    Precision Amplifiers Applications Engineering