730kHz Active bandpass filter
730kHz Active bandpass filter
(OP)
Nasically i currently have a design working on MultiSim (if anyone knows it) where i have a very nice 12kHz bandwidth around a centre frequency of 730kHz. The filter itself is a 4th order Biquad configuration with two stages, each with 3 op-amps in a feed-back configuration. Even the parts in simulation are the parts i'm also using in real life, Burr Brown/TI OPA658P, a 900MHz current-feedback op-amp (900 MHz is to accomodate future uses). However, when i wire them up on a test board, I get oscillations on the output. I realise that there may be stability issues with the feed-back, however only negative feedback is used. No oscillations are noticed in simulation and when i hook up a simple inverting amp, it works ok. Anyone with any tips as personally i'm at my wits end, cant find anything i can test any further. Any help would be gratefully appreciated.
Thanks,
Frank
Thanks,
Frank





RE: 730kHz Active bandpass filter
Just a thought!
RE: 730kHz Active bandpass filter
RE: 730kHz Active bandpass filter
RE: 730kHz Active bandpass filter
You are attempting to use a current-feedback opamp in a voltage-feedback situation. Your simulator doesn't know the difference.
You probably have the part on a perf-board or some type of dead-bug arrangement. If so, you can bet your sweet bippy that it is going to oscillate. You have all kinds of parasitic positive feedback paths.
Try selecting a voltage-feedback opamp with a gain bandwidth of no more than 10 MHz and using it. I think you will be much more satisfied with the results.
I hope this helps.
Lewis