You still don't know where is the noise situated compared with a given fundamental frequency. Are you sure you don't have harmonics? I'm suggesting again that a programable filter can do worse if you don't know where is the noise (in frequency domain) You need a spectrum analyzer or a good oscilloscope with FFT for measuring that. The error you get can also be a comparator offset which is variable with the input amplitude. If you need precision you also need a very good comparator. On 1/18/09, fred jones wrote: > > Thanks, I built his circuit 3 times thinking I must have done something w= rong (this is represented to be a working design) changing up the PCB layou= t but the same noisy results every time. His circuit was more noisy than t= he version I came up with. He and another guy partnered up with a website = of their progress and it hit me that they were up to rev11 on their circuit= s and PCB layouts because they were having noise problems. As you see in h= is article, they finally resorted to some very complex methods for extracti= ng the frequency because it was suppose to be more immune to noise. I thin= k their amplifier design may not be the best and they tried to compensate f= or it by extravagant programmable filters and software techniques to extrac= t the frequency in their latest design. > > > > Sounds like a proton precession magnetometer, or something very similar= .> You should check out James Koehler's article in Circuit Cellar:> > http:= //www.dtweed.com/circuitcellar/caj00202.htm#3455> > This was also his entry= in a contest, and additional information can be> found here:> > http://www= .jandspromotions.com/philips2005/Winners/AR1746.htm> > Basically, he used a= matched pair of low-noise transistors as the first> stage of his different= ial amplifier.> > -- Dave Tweed> -- > http://www.piclist.com PIC/SX FAQ & l= ist archive> View/change your membership options at> http://mailman.mit.edu= /mailman/listinfo/piclist > _________________________________________________________________ > Windows Live=99: Keep your life in sync. > http://windowslive.com/howitworks?ocid=3DTXT_TAGLM_WL_t1_allup_howitworks= _012009 > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist