That was a bit of a journey. I have to Couple to the filter using a capacitor as i need to block any DC from the 100 Ohm resistor, and because when i use a capacitor to feed the filter it forms a high pass filter which would be no good, now assuming our minimum input impedance is 10K and hopefully more i had to use as large a coupling capacitor value as possible, i have selected 1000uf Bi-Polar electrolytic giving us at 10K impedance. giving a -3db Frequency =0.02 Hz, assuming the impedance is much higher than 10K then this 0.02 Hz will be much lower. Now for the filter design. We wanted a gain of 100 or 40db, i set the -3db at 5Hz and -40db at 15Hz. Stage A 1st Order Low-Pass Stage B 2nd Order Low-Pass Sallen Key Stage C 2nd Order Low-Pass Sallen Key Design was optimized for low noise, i will screen over the components with copper foil. PCB has been designed and ordered. I'm going to plug this PCB company, 3rd time i have used them, $2 a PCB although P&P was $10, 5 PCB's for $18 inc P&P, they even provide the cad software which is via a web app, https://easyeda.com/
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