I built an 2nd order active highpass filter with cut-off frequency at 800Hz using an OPA847 (and R18=39.2k, R19=16.9k, R16=11.3k, R17=11.3k, C7=20nF, C8=20nF). Simulation works fine. In real life however, the active filter didn't work with these values. My assumption was, that OPA847 has a too low input impedance (2.7k), so I divided all resistors by 1k and multiplied the capacitors with 1k.
Now the oscillator shows me a square wave with frequency 450 KHz. It seems that the active filter is an oscillator now. How can I remove oscillation? Where do I have to look at?