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OPA333

H-O Martensson
Prodigy 30 points
Other Parts Discussed in Thread: OPA333

We have in a design connected a capacitor of 100pF directly on the input of the OPA333 believing this would improve the EMI behavior. We see on some samples instability on the output of the OP which we believe depends on the capacitor. Does anybody have any experience of an OP behavior when connection a capacitor directly on the input?

  • 0
    Guru 21975 points

    H-O,

    Can you please provide a schematic of your circuit. Additional capacitors on the input terminals can cause instability. It depends on the circuit details.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 30 points

    Hi Bruce,

    Yes, it looks like this:

    Any idea?

    BR

    /HOM

     

     

  • 0 in reply to H-O Martensson
    Guru 21975 points

    H-O,

    The combination of R43 and C43 will cause stability problems as they create a pole in the feedback path.

    I'm not sure of the purpose of R43. It is not needed in this G=1 circuit. Perhaps you are balancing the resistance at the two inputs to reduce the offset from input bias current. The input bias current properties of the OPA333 are not suited to this practice. I suggest that you eliminate R43 (make it 0 ohm). C17 is not needed.

    C43 is not likely to improve immunity from RFI. The OPA333 has internal RF filters dramatically reduce its susceptibility to RFI. If you find that the overall RFI rejection of your circuit is not sufficient, I recommend that you improve the shielding and filtering prior to the OPA333. The addition of 10pF capacitors at both inputs to ground located very close to the OPA333 may help but this is not certain.

    The output filter R44, C10 is a very low impedance load for the OPA333. I recommend that this be changed to 10k, 2nF.

    I hope these suggestions help.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 30 points

    Bruce,

    Thank you very much for your support and tip. We have successfully removed the C43 and got a much more stable output.

    If it has any effect on the EMC characteristics or not is still to prove but the stability is now ok.

    Regards, H-O.