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AD8422 for EMG Measurement

Hello,

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

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  • Hi David,

    Apologies as we have missed this thread.
    Were you able to solve your issue with your circuit?
    In case you still need help, can you provide the schematic diagram you're using with the AD8422? We can try to spot if there are obvious errors on the circuit. Also the actual part/model number of your electrodes will be helpful.

    With regards to the offset at the output of the AD8422, what's the gain configuration of the AD8422? Although you mentioned you have tried putting filters at the inputs already, it's good to keep in mind that most electrodes have inherent dc offset that will also get amplified.

    Can you also check if the voltages in the input of the AD8422 satisfies your specific configuration with this Inamp Diamond Plot , you can check out this video on how to use it. Please send out the schematic for our reference.

    Another thing on the offset and gain you can try to check out the circuit in the AD8237 datasheet and try to modify it for your application.

    Thanks David.

    Best Regards,
    Dann

Reply
  • Hi David,

    Apologies as we have missed this thread.
    Were you able to solve your issue with your circuit?
    In case you still need help, can you provide the schematic diagram you're using with the AD8422? We can try to spot if there are obvious errors on the circuit. Also the actual part/model number of your electrodes will be helpful.

    With regards to the offset at the output of the AD8422, what's the gain configuration of the AD8422? Although you mentioned you have tried putting filters at the inputs already, it's good to keep in mind that most electrodes have inherent dc offset that will also get amplified.

    Can you also check if the voltages in the input of the AD8422 satisfies your specific configuration with this Inamp Diamond Plot , you can check out this video on how to use it. Please send out the schematic for our reference.

    Another thing on the offset and gain you can try to check out the circuit in the AD8237 datasheet and try to modify it for your application.

    Thanks David.

    Best Regards,
    Dann

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