2 Replies Latest reply: Feb 13, 2013 11:02 AM by JBOsmont RSS

    AD8629 - Unwanted Oscillation

    JBOsmont

      Hi all,

      I am currently using an AD8629 amplifier for my analog input stages. Even if Spice simulations gave me nice results (AD8628 Spice Model), it appears that their applications went unexptectedly bad.

      Please refer to attached files for schematic applied.

       

      The choice of the AOP is due to this main characteritics

      - Very low offset voltage output

      - Wide GBW

       

      Here are the measures results (common for the 3 configurations application of the amplifier):

      -1V p-p oscillation at about 260kHz on output and inverted input - Non-inverted input seems stable.

      -If Output capacitor is removed : 1V p-p oscillation at about 260kHz on inverted input - ~100mV offset error on the output - Non-inverted input seems stable.

       

      I'd like to know what phenomen causes this oscillation. Is it a  design issue or did I choosen the wrong opamp for this application ?

      Have you any feedback on Spice model veracity?

      Secondly, I'd be glad to have any alternate opamp proposal.

       

      Thank you for you answers.

       

      Best Regards,

      Jean-Baptiste Osmont

        • 1. Re: AD8629 - Unwanted Oscillation
          harryh

          Jean-Baptiste,

           

            I'm not sure which comments go with which schematic, but several things.

          On the diff amp, C75 should be across R161.

          The very large resistors in combination with the op amp input capacitance gives another pole/phase shift.

          For the non-inverting, R267 is unecessary with picoamps of input bias current.

          The 10 ohm Riso is probably too small to isolate 1 nF.

           

          Harry

          • 2. Re: AD8629 - Unwanted Oscillation
            JBOsmont

            Hi Harry,

             

            Thanks for your feedbacks, I couldn't agree more on your improvements you proposed.

            Even if my oscillation issues had been stopped for a long time I had never been able to replicate the oscillations on Spice simulations. Even worst, a stability analysis shown up that phase and gain margin would be acceptable.

             

            Would it be possible that high impedance resistors have a higher influence on stability issues on amplifier inputs which has auto-zeroing and chopper-stabilized features like AD8629 than other usual amplifier like AD822 ?

             

            Best regards,

            Jean-Baptiste