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62.5kHz ringing Noise on signal
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syntax333:
I designed another PCB without ground plane. Can you comment on the design? Do you think there will be interference on signal?
StillTrying:
"Do you think there will be interference"

Quite likely, it could even be worse until you find where the 62.5kHz pulses are coming from.
syntax333:
It could be parasitic capacitance due to lm1881 and DG419 since all of the integrated circuits including OPA657 has same ground plane around their pins and LM1881, DG419 are probably switching according to 62.5kHz/4 = 15.625kHz as others have stated.
David Hess:

--- Quote from: syntax333 on May 08, 2019, 03:12:56 pm ---
--- Quote from: David Hess on May 08, 2019, 03:11:29 am ---The layout makes me think that there is excessive parasitic capacitance between the inverting input and ground lowering the phase margin of the operational amplifier causing it to oscillate.  This is a problem if the ground plane and flood fill is not cut away around the inverting input.

The impedance at the inverting input in parallel with the parasitic capacitance to ground creates another pole inside the feedback loop which lowers phase margin.  One solution to this is to add a compensating capacitance in parallel with the feedback network.
--- End quote ---

Ground plane is in B.Cu and opa657 is in F.Cu in my PCB board. Do I need to still cut out in back side of the PCB to lower the capacitance? I cut the PCB to isolate ground plane around opa657 however there is still noise?

Do I need to design PCB without ground plane?
--- End quote ---

No, but the place to start is to remove the ground plane from proximity to the inverting inputs to lower the parasitic shunt capacitance.  This is a common problem with high speed operational amplifiers.

As a test, add a small amount of capacitance in parallel with the feedback network.  Up to 10s of picofarads is usually enough.  This will cancel out any pole caused by the parasitic capacitance to ground.
syntax333:
Adding capacitor in parallel with feedback loop helps a lot but its not enough there is still distortion on signal.

So can we conclude that ground plane was the problem?
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