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I have this electronic circuit which is from a shield for the Arduino which allows me to collect ECG signals. However I do not understand what is happening. I am familiar with op-amps and amplifiers in general but do not understand this circuit.

There is a INA321EA (PDF) instrumentation amplifier and a OPA43040U (PDF), which I can find on the internet, however I would like an explanation of what is happening in the circuit so I can learn how each component works.

enter image description here

http://i.stack.imgur.com/zXL2A.png

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  • \$\begingroup\$ Is there supposed to be a circuit there? \$\endgroup\$
    – akellyirl
    Mar 3 '14 at 16:54
  • \$\begingroup\$ @akellyirl There are supposed to be links to datasheets too. \$\endgroup\$ Mar 3 '14 at 17:02
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The first op amp sets up a reference at Vcc/2 for single supply operation. The resistors on the input limit current to the body and set up a body reference at or near the common mode voltage on the electrodes (using big noisy resistors). The instrumentation amp takes the differential signal, provides a gain of 5, and spits it out as a single-ended output, where the next op amp low-pass filters it, and then sends it to an inverting amp with a big gain (1M/3.3k) that also removes most of the DC offset prior to amplification.

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  • \$\begingroup\$ Why use the low pass filter here? Also I cannot see the inverting amp. \$\endgroup\$ Mar 7 '14 at 11:00
  • \$\begingroup\$ Actually I see it, but why the 4.7 nF capacitor (C4). \$\endgroup\$ Mar 7 '14 at 11:09

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