EMG design help using AD8221 INAMP
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.