Filter, o Filter… where art thou?

December 02, 2007

Filters… I’ve always had an aversion for analog filters especially the multiple stage Butterworth types… So, i thought i’d try to use an alternative. Enter LTC1063… a 5 stage Butterworth filter all included in a single chip that only requires an external RC to set its cutoff frequency. I was so excited at first, coz I thought this chip would solve all my problems. Well, it would have, only if my operating frequency wasn’t so low. Imagine using this chip to drastically reduce every frequency from 50 Hz and above. The clock frequency has to be ten times that… and unfortunately, that falls within the audible range. What that means is that the clock frequency is bound leak into the circuit and guess what? It did!! I would’ve had to have added another butterworth filter just to remove the clock frequency itself. So, i shoved LTC1063 aside and decided to revert to the old-fashioned method…

Back to the 5 stage butterworth Sallen-Key stuff… Played around with different variations to see if I could get a decent result. My aim, as stated before, is to simply reduce all the frequencies above 50 Hz. Not really interested in experimenting with High Gamma brainwave frequencies as of now. I managed to arrive at a good Sallen-Key setup… will post the component values and the calibration scope results in a while. Until then, Ciao!

EDIT: I found a better filter topology called the FLIEGE FILTER. It’s amazing to find free info like this from Texas Instruments. I’m now using this filter in my circuit (check out the schematic in one of the posts above )


~ by teknomage on April 27, 2008.

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