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OPA656: OPA656 oscillation

kent gu
Intellectual 685 points
Part Number: OPA656


Hi,

Have the below circuit with OPA656, without or with input, have the below interesting issue.

       ----30M oscillation when C34 is 220pf, Oscillation disappear when C34 is 12pf. Why? Change R35 can't help.

      --I select other FET OP, there is similar oscillation as ADA4637 when C34 220pf. I change OP into no FET OP, there is no oscillation even C34 220pf.  Why?

Best Regards,

Kent

  • 0
    Guru 48395 points

    Morning Kent, 

    To pursue would need to the source and load impedance - or are they just open, if so, you might try putting a 100ohm load to ground on the output, 

    Also, the -1V negative supply might not provide enough I/O headroom, may need to increase that to -2V to satisfy the output headroom of 1.7 volt worst case on the OPA656. 

  • 0 in reply to Michael Steffes
    Intellectual 685 points

    Hi Michael,

    Thank a lot for your reply. 

    yes, I tried input of OP, and pull down 100ohm for OP output, still same oscillation.  Also tried ADA4637(FET) with 5V/-5V, same issue. 

    As I mentioned, I tried no FET OP, there is no oscillation, so I think may be related to FET OP of lower bias current, and cause the oscillation.

    Best Regards,

    Kent

  • 0 in reply to kent gu
    Guru 140200 points

    Hi Kent,

    please show the full schematic and a photo of the layout.

    Do not directly touch the output of OPA656 with the scope probe but insert a 50R resistor which you mount close to the output pin of OPAmp.

    And use such a scope probe:

    Kai

  • 0 in reply to kent gu
    Guru 48395 points

    Well I did try some loop gain phase margin sims, the original 2002 model in the V11 library shows quite high xover with good phase margin

    The updated 2016 model show more reasonable crossover with about 40deg - so neither indicate a problem. What non FET input device did you try? what is your frequency of oscillation?