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The problems of AD623 used in right leg driven circuit


I want to use DRL in collecting EMG signals, but now i meet some problems. It is clearly that there is only bottom noise when input the vacancy. And we can get EMG signals including power frequency noise when the input is plugged into the body. So a right leg dirven is needed to decrease it. Then i design the DRL circuit by referring to the datasheet of AD620(by the way, there is no DRL circuit in application of datasheet of  AD623). I use 3.3V single power supply and the dc voltage of gain setting resistor is also 3.3V, which results in DRL circiut output voltage saturation. And the output of total circuit is the similar to when input the vacancy. that is noly bottom noise. I want to know what is wrong with my circuit and how to deal with the dc voltage in gain setting resistor(a big dc voltage can result in DRL output saturation).

Thank you in advance. 

  • Hi YuxiangRousi,

    Good day!
    Apologies we missed this email thread, in case you haven't figured out the issue with your circuit, we're here to help out.
    For this question, will it be okay to share the circuit diagram you used?  We may be able to spot an error. and may I clarify what you mean when you say "when input the vacancy"? What's the actual gain setting resistor you used? Since you also mentioned that you're saturating the output, electrodes usually have inherent DC offset around 300mV and thus we may be limited to the gain we can set as we can easily saturate the Inamp's output. But I think providing the schematic diagram you used will be the best way, I'm interested to know how you connected the right-leg feedback circuit out of the AD623's RG pins.

    Also, you mentioned that you used the AD620's circuit have you checked out this article? I would think that the front-end will be close to the EMG application too ECG Front-End Design is Simplified with MicroConverter® | Analog Devices. You may be able to spot what's wrong with your circuit or you can get some information on how to make it work.

    I'm also interested on the scope shots of your system, it will be great if you could share them in detail.
    Thanks YuxiangRousi.

    Best Regards,