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): And looks like this when enlarged, is a 10MHz sine wave.
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: The input op amp is configured like this: And here's the codec's datasheet.