Op Amp Amplifier Chaining - Oscillations & Coupling

[0xDEADBEEF]

I am trying to chain two op amp amplifiers. I thought that I could couple them through a capacitor so the output offset of the first amplifier doesn't get amplified. Now the first one is oscillating because of the capacitive load. Is there a way to do ac coupling without using a capacitive load for the first amplifier, or another way to prevent the oscillations?

 

[skeptic2]

1. What is the frequency of oscillation?
2. What is the gain of the op amp circuits?
3. What is the frequency range you are amplifying?
4. As always, a schematic will help.

 

[0xDEADBEEF]

1.) 8MHz
2.) 10000
3.) 20kHz - 100kHz
4.) G=100 Instrumentation amp followed by a G=100 inverting amp followed by a sallen key 100kHz low pass.
I haven't really found the source of the oscillations, but driving capacitive loads is discouraged, so I wondered if there is any way around it.

 

[Averagesupernova]

I am trying to chain two op amp amplifiers. I thought that I could couple them through a capacitor so the output offset of the first amplifier doesn't get amplified. Now the first one is oscillating because of the capacitive load. Is there a way to do ac coupling without using a capacitive load for the first amplifier, or another way to prevent the oscillations?

A coupling capacitor is not generally considered a capacitive load.

 

[Topher925]

I know this is obvious, but have you tried using a smaller ceramic/polyfilm capacitor?

 

[0xDEADBEEF]

I found the bug: Ironically the low pass was doing the MHz oscillations. The cable on the output was enough of a capacity! 20 Ohms in series and the problem is gone...