I am trying to build an EMG amplifier using INA121 single supply, but I have run into numerous difficulties.
My circuit is below (using three electrodes including RLD). How I think it works: The common mode voltage is detected, and sent to the right leg drive. DC level is removed, then AC coupled CM noise is inverted and sent back to the patient through the third electrode. Also, since INA121 requires the inputs to have a CM voltage of 2.1 V above ground, the RLD output should have a DC level of 2.5 V
Unfortunately, this circuit doesn't work when constructed, in that I do not see my EMG signal, nor does the noise get rejected. The +/- inputs don't have a correct DC level, only 1 V or so. I am guessing it has to do with tolerances/offset voltages. Can anyone suggest a fix to make it practical?
NB: I also get much better performance when I connect the ground electrode directly to Vcc/2.
The INA121 has very limited input common-mode range when operated on a single 5V supply. See the graph in the lower-right of page 4 of the data sheet. This graph shows that the very limited range (dotted lines) are not centered at 0V common-mode voltage on +/-2.5V supplies.
You would find it much easier to operate the INA121 from higher power supply voltage. You may be able to get an operating system by moving the common-mode voltage slightly higher by changing your pseudo-ground voltage divider circuit.
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