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I am using this Differential In/Out Opamp circuit as ADC driver and to provide some gain(G=6.4).

The component values are R53,R52=100, C28, C27=1.1uF, R56= 1K, R57=open, R55,R54= 2K7, U6=OPA2211.

enter image description here

At ADC input I have simple RC filter. enter image description here

Component Values are C10,C11=Open, C7=1uF, R26, R27=100.

Now the problem I am facing is My opamp output seems to have some noise like this, enter image description here

But as I expanded the time scale I saw this, enter image description here

I don't understand where this is coming from, I powered my ckt with linear supply but saw no improvement. Sensor connected to OPamp Inputs is a wheatstone bridge Pressure sensor and it looks cleans. kindly help me.

Edit: I am using MCP1640 Boost converter to get 5v from 1 cell Liion. And using MCP1702 to get 3.3 V from 5V. I thought it was switching noise but I also tried with Linear supply.

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  • \$\begingroup\$ What's the peak to peak voltage of these spikes? \$\endgroup\$
    – Abdella
    Commented Aug 22, 2015 at 13:00
  • \$\begingroup\$ 120mV................................ \$\endgroup\$
    – Sajid
    Commented Aug 22, 2015 at 13:06
  • \$\begingroup\$ The first image shows 40mV and the second shows 120mV. Which one shall we pick? \$\endgroup\$
    – Abdella
    Commented Aug 22, 2015 at 13:10
  • \$\begingroup\$ Hmm Even I don't understand it. consider 120mv one. \$\endgroup\$
    – Sajid
    Commented Aug 22, 2015 at 13:15
  • \$\begingroup\$ Anyway, what are the voltage regulators used in your circuit? please write their part numbers. \$\endgroup\$
    – Abdella
    Commented Aug 22, 2015 at 13:25

2 Answers 2

Reset to default
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I found the culprit. Actually it is due to ADC's switching when it takes Sample. I disconnected OPamp's Output from ADC by removing the R from RC and the ringing was gone.

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  • \$\begingroup\$ Does proper decoupling of the op amp and adc fix this? \$\endgroup\$
    – Scott Seidman
    Commented Aug 24, 2015 at 13:00
  • \$\begingroup\$ No it has nothing to do with decoupling. When ADC's switch closes to take the sample(i.e. charge ADC's Internal sampling capacitor) then there is a disturbance in signal path. Increasing value of Resistance did suppress it a bit, but it is still there. \$\endgroup\$
    – Sajid
    Commented Aug 25, 2015 at 7:08
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The 0.1uF is a too high value to be driven by the opamp. Remove them, or replace by pF capacitors. Ampops naturally use to be unstable when driving capacitive loads. The best thing to do is build the filters at the input or feedback traces.

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  • \$\begingroup\$ "Ampops" is a new one, Jair. Welcome to EE.SE. \$\endgroup\$
    – Transistor
    Commented Feb 13, 2020 at 21:30

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