From: Tom Handley > site to check out the circuit and the first obvious problem is that filter > is NOT a `band-pass' filter, but a `band-reject' filter!... Are you sure? The book I followed says it's a band pass configuration and the bode plot shows clearly a small pass region just where I calculated it, with continuous attenuation outside of that, and a small notch hump at some frequency higher than my pass region which looks like a typical chebyshev to me, and the lab test validated that I obtained my highest output in my pass band frequencies and attenuated from there. I re-ran my spice simulation with motorola op amps (they seem to not load down the output like the TI ones did, thus giving me the calculated output levels). I used the MC33172 model from Electronics Workbench and then exported the spice file to run in Orcad, which I prefer for spice over workbench scopes and bode plotters....and with a 1v peak input sine, I got about 25mv out at 300Hz, which if I calculate properly, is around 32dB attenuation, which looks relatively like a flat line but it's within my design specs for having at least 20dB attenuation below about 310Hz. Then I ran the simulation at that 310Hz corner and calculated almost 19dB attenuation which is close to design spec. > Also, there is > no need to have three pots for offset. Just use one and a voltage follower. Yeah I've already reduced those pots down to the one reference. I mostly put the 3 in there for the purpose of clarity to indicate I'm biasing all 3. I've revised that schematic many times. > Out of curiosity, I ran a SPICE simulation with a Bode plot on the first > stage of the filter. My tests were the entire 3 stages and I got the same type of frequency response no matter what op amp I chose. My only issue was they attenuated differently. Someone suggested the problem with certain op amps was loading on the output, including the feedback loops, and so I'm placing another order today to get those motorola ones that worked well. If not I'll just try whatever I have laying around until something works. I've added the spice file for the whole filter with the motorola op amps on that page. The direct link is http://rivendell.fortunecity.com/perilith/831/filter.cir if you wanted to look at it again. I'm sure it's a band pass. > I would say, in general, if you go beyond a 2'nd order filter, you should > look at an alternative to Op-Amps unless you have spares in a given package > and/or find a price advantage. If I could do this over again I would have a completely different approach than I ended up with but right now time is what is against me and I have a lot of calculations and demonstrations I can "show" for what I've done so it's adequate for my purposes for now. If only I could find time to work on the pic code again. I'm always seemingly too busy to do PIC things... I guess I'm doing my usual habit of underestimating what's involved with gluing code together to have a successful program based on individually working math routines and port accessing.