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AFE4300 with body connected to ground

ccent
Intellectual 340 points
Other Parts Discussed in Thread: AFE4300

I am using the AFE4300 for a body impedance measurement device. The device seems to be operating fine with a few LSBs of noise when using FWR mode. I am also using ECG on the body and have a lead connecting the body directly to ground. I noticed that when this lead is connected to the body, the AFE4300 measurement changes slightly and becomes much less noisy (~1 LSB of noise). I also noticed that if I remove one of the two Iout electrodes and connect ground, I can still obtain an impedance signal. I was wondering a few things:

a. Is it possibe to use a ground/RLD electrode while using the AFE4300 for impedance measurement?

b. Why does a separatore ground electrode connection to the body clean up the impedance signal? 

Any light you can shed on this matter would be greatly appreciated.

Thank you!

  • 0
    TI__Mastermind 18285 points
    ccent,

    We have received your post and hope to have a response back to you soon.
  • 0 in reply to Amy Schnoor
    TI__Mastermind 18285 points

    ccent,

    Sorry for the late reply. Can You give us a screenshot of your signal with and without one of the IOUT electrodes tied to GND. Also I have included a screenshot image of what I believe you IOUT circuit looks like. Can you verify this is what you have?

  • 0 in reply to Amy Schnoor
    Intellectual 340 points

    Thanks for the reply. In terms of noise, the difference is really only a few LSB. But for my application, this is important. The setup is similar to what you have drawn, except that the ground is connected directly to the body through a separate electrode. I have five electrodes on the body: Ioutp, Ioutn, Vsensep, Vsensen, gnd. When I have the gnd connection (for ECG CMRR), the impedance is more stable and less noisy. However, I have also noticed that the impedance value changes significantly. For example, without the ground electrode, I obtain an impedance (adc value) of 6200 and with the ground I obtain a value of 10200. Is it possible that the current is flowing to ground through the body somehow? I am not understanding why the ground is changing the impedance value. Perhaps it's affecting the current drive circuit?