How to acquire two leads in the same time for ECG design?

[dunadan02]

I am building an ECG and my problem is: How can I acquire 2 leads in the same time?
The things I want to do is to calculate the third lead given by the formula : Lead III = Lead II - Lead I. First of all, I have to acquire the first two lead and then substract them.
I am using the circuitry given in the AD620 datasheet to acquire one lead + filters.
My first idea was to build two differential amplifier (two AD620 circuitry) but I think there will be a problem with the right leg drive.
Any idea?

Thanks!

 

[odiseo]

right leg driven circuit

For the right leg you can just add the signal from all the input leads, invert it and apply it to the right leg. It is called active right leg circuit.

Other choice is just to apply gnd or just a mid point voltage from your input ISOLATED circuit to the right leg. It wont give the best performance for the CMRR theoretically but it will work just fine

 

[akss_here]

right leg drive ecg

very similar circuit related case i found this site .. with use of ane AD620 amplifier..

refer the paper DRIVEN Right leg drive