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[Reply #15 on:]

Found such circuit, based on LM358N , what do you think?

[Reply #16 on:]

Thanks for the tips! Any ideas what kind of amplifier and filters would be better?

What voltage are you trying to record, and within what frequency range?

[Reply #17 on:]

An Electrocardiogram Amplifier does a very specific job and has been studied a LOT. Start with Google "Electrocardiogram Amplifier"

Read some of the work that has already been done, such as:

"http://www.rle.mit.edu/acbs/publications/journal_papers/micropower_ekg_amp.pdf

...it's not a good subject to start from Zero Base on.

[Reply #18 on:]

what kind of amplifier and filters would be better?

you may need an instrumentation amplifier stage to ensure low voltage signal is amplified providing minimum attenuation to actual signal.
heart rate is usually upto around 200 bps-250 max right? so any thing above the frequency could be removed
for signal amplification you may employ a high gain stage using opamps (i assume you observe the waveform on a CRO to get an idea of the range of amplification needed) regards

[Reply #19 on:]

Thanks for the input guys. From what I learned so far, guess AD620 chip would be the best, what do you think?

[Reply #20 on:]

I would use an op-amp with FET inputs such as this dual one that can be run from a single +5V supply http://www.farnell.com/datasheets/76675.pdf. Or you could use the widely-available TL081, but then you would need to provide a -5V supply.

EDIT: I just read the following in that article I linked to: "Use instrumentation amplifiers with very high common mode rejection ratios on the order of 100dB".