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Using multiple AD8232 electrodes with a common reference electrode?

Question asked by dmeglan on May 7, 2018
Latest reply on May 23, 2018 by jrombola

I am working on a design that has a number of electrodes connected to AD8232 chips which use a common reference electrode. I am aware of the ADAS1000 series chips but for my purposes where there can be an array of dozens of electrodes, the AD8232 seems the more scalable choice. I've prototyped 2 and 3 electrode designs derived from the reference designs in the documentation and concluded that I can do this without needing the do the right leg drive, so it's 2 electrodes per channel where one of the electrodes is the same for all channels to give them a common reference point.


My question is does this globally common electrode need to be buffered separately at each AD8232 chip or can its signal just be passed from chip to chip? I had assumed that it needed buffered but now I am having second thoughts as I try scaling the design.


I tried a simple single supply unity buffer using a ADA4692 but I am not getting a proper signal out of that. I am thinking I may have a common mode voltage issue or being to close to the supply rail. I am using a battery supply at this point, to simplify grounding/isolation, and that is around 3.0-3.3V. The signals on the reference electrode should be in the range of 300mV due to the half cell potential of the electrode contacting the skin. I believe this is at the edge of the amp spec if I am reading the data sheet right. I know I can bias the signal at the mid-point of the supply to make the op amp work but then I have to pull it back down on the other side of the unity buffer before it gets to the AD8232 to make it work with the other electrode connected to each AD8232.


Suggestions? Is it possible to just connect a single reference electrode across a bunch of AD8232 chips? I have obviously not tried that yet. I had assumed the impedance at the input to the AD8232 would cause issues if there were dozens of them in parallel connected to the same signal.


Thanks for insights here