Mauricio Jancic wrote: > I need to find the transfer function of this filter, whats the > topology name? A more detailed picture can be downloaded from > www.janso.com.ar/filter.jpg It breaks down into fairly small pieces, but the combination is rather bizarre! The key is to realize that the outputs and the inverting inputs of the opamps can all be treated as voltage sources, which nicely isolates each R-C and L-C pair. To start with, the response depends heavily on the source impedance of whatever's driving the input, given the 5.7 nF directly across it. The first stage has a gain of 11, with a rolloff at 13 kHz because of C2. L1/C10 form a basic low-pass filter, but they resonate at 50 kHz, and with little damping, they will produce a huge peak at that frequency. The next two opamps are essentially identical. They'll have unity gain at DC, with a response that begins to rise above 16 kHz and levels off again around 33 kHz because of the 2.2 nF capacitor. But the series-resonant LC circuit in the feedback path will again produce a huge peak at 50 kHz by basically shorting out the feedback at that frequency. The R9/C8 combination produces another low-pass pole at 33 kHz. The output capacitor C4 blocks DC, creating a high-pass pole whose frequency depends on the load impedance. If I had to guess, I would think that this is the front end to an IR or ultrasound remote-control receiver, where there isn't much concern about ringing at the resonant frequency. -- Dave Tweed -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist