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There's some thing wrong with my audio codec. Unlike the modern codec, its input gain is configured externally with op amp's resistor. However with unity gain, I see strange wave form on the input amplifier's output(AO pin), like below(My input signal is a DTMF signal): enter image description here And looks like this when enlarged, is a 10MHz sine wave. enter image description here

I was thinking that the probe capacitance is causing this, because firstly I use passive probe which has about 19pF input capacitance. However, after I changed to an active probe(Cin in less than 1pF), which measured the above picture, the problem is still there.

I occasionally switch the passive probe to 1x with 90pF Cin, then there's no problem on output signal. And it is also OK when I add an 82pF output capacitor for op amp output terminal.

So why my input op amp's output is not stable without the adequate capacitance? Thank you.

Here's my schematic: enter image description here The input op amp is configured like this: enter image description here And here's the codec's datasheet.

iouzzr
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    You don't even tell what opamp you are using – PlasmaHH Apr 10 '18 at 14:04
  • What do you see on A0? – Andy aka Apr 10 '18 at 14:06
  • @PlasmaHH the op amp is inside the codec. – iouzzr Apr 10 '18 at 14:24
  • @Andyaka AO is the first and second picture. – iouzzr Apr 10 '18 at 14:25
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    You say in your question "I see strange wave form on the input amplifier's output(AO pin)" then you talk about an op-amp and refer to this as "The input op amp" hence, that is very confusing. Show the full circuit, name the op-amp. – Andy aka Apr 10 '18 at 14:31
  • @andy this audio codec has input and output amplifier, for microphone input and speaker output. So input op amp means the op amp used for microphone signal. – iouzzr Apr 10 '18 at 14:39
  • Well, your opamp has a feedback and any system with a feedback can oscillate. There's most likely something that stimulate the oscillation (like a clock) combined with a resonant circuit. Opamp usually includes capacitance to reduces this effect but it may not be enough in your case. Usually the solution to this type of problem is to filter out the hf component in the feedback loop (like adding a capacitor like you did) or moving the resonant frequency where it won't be stimulated (changing opamp or wire inductance) – Pier-Yves Lessard Apr 10 '18 at 14:45

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