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OPA855: OPA855 with harmonic

Eddie Chou
Expert 4710 points
Part Number: OPA855

Hi Team,

My customer test OPA855 with schematic below,

Customer find output harmonic if no signal input, customer try change Rout resistors value and harmonic also changes,

Rout 0ohm

Rout 50ohm

Rout 100ohm

we suggested customer short the input and harmonic disappeared. 

Would team help comments if any reason might cause the harmonics? thanks.

Thanks & Regards

Eddie

  • 0
    TI__Mastermind 22270 points

    Hello Eddie,

      The harmonic measurements were taken with input floating or terminated to a certain impedance?

      When suggesting shorting the input, does this mean both IN- and IN+ are terminated to ground?

       Also, is R784 not populated?

    Thank you,

    Sima

  • 0 in reply to SimaJalaleddine
    TI__Expert 4710 points

    Hi Sima, 

    The harmonic measurements were taken with input floating and measured by spectrum analyzer. (before R780, R782 is floating) 

    Input short is before R780, R782. R784  is not populated.

    Thanks & Regards

    Eddie

  • 0 in reply to Eddie Chou
    TI__Mastermind 22270 points

    Hello Eddie,

      The gain changes between the two configurations. With floating inputs, the gain will be lower than when shorted prior to R780 and R782 or terminated to 50 ohm connectors. Since the amplifier is decompensated, a minimum gain of 7V/V is required for the OPA855. Also, unusual behavior might be seen due to floating the inputs.

    Thank you,

    Sima