AD8422 for EMG Measurement


I am making a small board for EMG measurement placed on the face skin. To achieve this I use the AD8422 as an instrumentation amplifier. There will be 2 board and therefore 2 EMG channels that will then be processed by ADC on the motherboard. The electrodes will be stainless steel electrodes with a diameter of 1 cm separated by 1 cm. After realization of the prototype, I notice several problems:

• The appearance of an offset on the output of the amplifier AD8422, I tried to make filters pass high on each input, to add a capacitor in series with the resistance RG but it does not work. I also tried to create a RLD circuit for the reference electrode but without success (the RLD circuit does not seem suitable for a multi-channel system). I don’t know if the problem is an impedance mismatch of the electrodes, so I added serial resistors to the amplifier inputs but without success.

My reference electrode is currently connected to the ground of the system, but it doesn’t seem to work to cancel out the common voltage of the skin.

I see several circuits on the potentially functional internet, but despite my tests I can’t find the one that would be functional with the AD8422.

Thank you for your help