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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.

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    \$\begingroup\$ You don't even tell what opamp you are using \$\endgroup\$
    – PlasmaHH
    Apr 10, 2018 at 14:04
  • \$\begingroup\$ What do you see on A0? \$\endgroup\$
    – Andy aka
    Apr 10, 2018 at 14:06
  • \$\begingroup\$ @PlasmaHH the op amp is inside the codec. \$\endgroup\$
    – iouzzr
    Apr 10, 2018 at 14:24
  • \$\begingroup\$ @Andyaka AO is the first and second picture. \$\endgroup\$
    – iouzzr
    Apr 10, 2018 at 14:25
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    \$\begingroup\$ 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. \$\endgroup\$
    – Andy aka
    Apr 10, 2018 at 14:31

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