ECG Shield electronic circuit explanation

I don't see any unresolvable pin conflicts, so it should be safe to try.

• The GPRS shield uses the UART and some digital pins, but none of the A-marked analog pins.
• The ECG shield uses some of the power pins for power and ADC reference voltages, and A1 thru A6 (selectable with a jumper) for the analog ECG signal back to the Arduino. Either pin D4 or D9 is used for a calibration signal, this is selectable by a jumper. Make sure you select D4, because D9 is used by the GRPS board!

Of course, other factors (such as software conflicts between the Arduino libraries for both shields) could ruin your day. I can't promise it'll work as you intend it to.

Your real challenge will probably be in processing the analog ECG data into something meaningful that can be transmitted over GPRS (determining heart rate, detecting complex events like arrhythmias etc.). You probably shouldn't send the raw ECG waveform over GPRS, as continuous transmission takes a lot of power and will likely yield a large bandwidth bill. Depending on your situation, it would probably be much more efficient to send a digest of interpreted cardiac parameters periodically (and only sending raw waveform data upon request for example).

Hopefully needless to say: do not use this for any health-critical purpose. Neither the Arduino nor the shields and other hardware you're using have been specifically designed or certified for healthcare purposes. Experimenting with ECG equipment is great fun for education and perhaps some sports performance tracking, but you should never consider your project as replacement for seeing a physician and following his/her advice.

For question 1: \$V_S^i\$ will appear at the negative terminal of OA2. At high frequency, the impedance of \$C_b\$ is low, so the voltage on both sides of the diodes approach the same value, thus this is no current, an effectively high input impedance. For low frequencies, input that differs by more than a diode drop from \$V_{bias}\$ will cause current through at least one of the diodes.

For question 2: For true common mode signals, \$ V_s^i = V_{cm}\$. Thus, \$V_{cm}\$ also appears at the negative input of OA1. With the same voltage at both ends of the series combo of Rf and Cf, there is no current.

For question 3: I think this may be the same answer as for question 1, with the point just made a slightly different way.

Question 4: As long as there are no saturations -- yes, and that is central to the analysis of the circuit.