Hello all,
I'm having a weird problem with a Sallen-Key filter. I've build a filter which should have a cut-off frequency around 50MHz. I'm using a current-feedback opamp which has a 1GHz bandwidth. The problem seems to be in the feedback capacitor. The circuits acts like if the feedback capacitor is twice the actual value so the pole is in the wrong place.
If I put a 33pf capacitor in the real life circuit a simulation which uses a 66pf feedback capacitor gives me the same results as the real life circuit. I also tried a 68pf capacitor.
The filter is driven by a unity gain buffer. I'm using a 1:20 probe with a 50 Ohm output (passive divider) on an oscilloscope with a 50 Ohm input. The signal going into the filter looks fine. The whole circuit is on a 4 layer PCB.
Does anyone have some thoughts?