Electronic – EMG design help using AD8221 INAMP


I'm designing active electrodes for surface EMG, active electrodes being electrodes directly connected to a pre-amp and filtering stage on the measurement site.
Im working with the AD8221 IN-AMP to pick up the differential signal and I am trying to eliminate the need for a reference electrode by driving the reference pin of the IN-AMP via a voltage divider and buffer to bring the AD8221 to midsupply level.
However with my simulations I have failed to receive the desired midsupply voltage level at the reference pin and infact only get a few hundred millivolts. Can someone please explain to me the theory of operation with the reference pin? and whether this method will effectively eliminate the need for the reference electrode?

Circuit specs;
+/- 5V power supply for the entire circuit.
Gain = 100. (Rg = 499 Ohms)

I have read through the other threads related to my topic, but none have encountered the problem I have. I am running a simulation on MultiSim v.13

Update: the current schematic

enter image description here

Best Answer

The reference pin has a 20k ohm input impedance and your potential divider may be formed from large resistors i.e. values in excess of 10k - this will cause a problem. Try driving the ref pin with a voltage source. Also note that there is a leakage current from this pin of about 50uA. This will produce a 0.5 volt error when fed from a voltage source with a 10k impedance.