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-2mV ~ 2mV signal

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Hi everybody, that's my first post here!

I need to measure a ECG(ElectroCardioGraph), but the signal are to small(About 4 mV total range) and low frequence(1~200Hz).

I realy dont know how to do it.... :'(

I think that the steps are those:

1 - Measure de signal
2 - Pass into a filter(LowPass with AmpOp)
3 - Amplify with AmpOp
4 - A/D convertion
5 - MCU processing

I need to know how to do the steps 2 and 3.... ??? With Op Amp to use and maybe some examples.

I'd like to know if any boby here already have this problem.

I'm from Brazil, and here some Instrumental Op Amp are too expensive, I'd like to use cheap Op Amps(Like 741, 358, 353,etc)...I really don't need a lot of precision....

Other information, my supply will be only one 9v battery..... :(

Thank you in advance,


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You need to gain first, then filter. The filter should be simple construction, not to say that you couldn't use a capacitor around the opamp like a state variable filter. You can use many stages so that each stage will properly handle the amplitude of the signal which is continuing to be amplified. I might stay away from opamps because you don't need the differential inputs. All you need is a high gain pushpull with arbitrary impedance.

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hello tercio,
i really wonder, your supply is 9v and the signal goes below zero,
how to use op-amp in amplification, you need volatge divicer in series with your input to shift the negative voltage over the x-axis and make it postive,

will this suite you project, to have a whole postive signal.

explain plz

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