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AFE4300 calibration

Other Parts Discussed in Thread: AFE4300

Hello,

I am following the tidra02 reference design available on your website and I want to built a custom board with as few components as possible. 

If I want to use the calibration, do I need the 10M resistors R63-66, the 1uF C27-30 and the 0.1uF C5-8 capacitors in tidra02? What is the purpose of these?

I guess I only need R57 and R58 for the calibration. Please verify!

Thank you,

Mastro.

  • Hello Mastro,

    The purpose of the 10M resistor is to provide a DC feedback path for the op amp

    The caps are to provide low impedance path for the excitation signal and high impedance for DC currents (patient safety).
  • Hi Praveen,

    Thank you very much for your prompt reply.

    1. So none of these capacitors are needed for the calibration, since they are not connected to the patient and only to the on board test resistors?

    2. The AFE4300 datasheet mentions nothing about any DC feedback 10M resistors required at those pins. I think they are also for patient safety, in which case they can be omitted from the design???

    Can you please double check this with me?

    Thanking you in advance,

    Mastro.
  • 1. The capacitors are not needed for calibration.
    2. There is an op amp inside the AFE4300 that drives the excitation current into the body. The external electrical network between the active IOUT pins (IOU0 and IOUT1 for example) serves as the feedback network for the op amp. While the excitation signal is AC, for the op amp to work in a stable manner, it needs a mechanism that sets the DC feedback. Our earlier recommendation was to use a 10MOhm resistor between IOUT0 and IOUT1. However this was found to be unsatisfactory from a DC feedback perspective. That is why we changed our recommendation to add a 10 KOhm resistor in parallel with the 1 uF caps. The two series 10 KOhm resistors in series with the resistive component of the body impedance will now be the mechanism that sets the DC feedback for the excitation op amp.
  • Hi Amy,

    Thank very much for your reply.

    What kind of issues will I have if I stick with the original recommendation (the 10MR)?

    I am awaiting an answer to a number of question in another post of mine and I would be extremely grateful if you could get back to me on those question too:

    e2e.ti.com/.../1521773

    Thank you,

    Mastro

  • Mastro,

    You may see some DC feedback issues. I recommend switching, but if you are using large size resistors you can always switch them out in the future.

    I will respond to your other post soon.
  • Hey Amy,

    I'm currently also building a custom circuit with AFE4300 can you please elaborate on the 10 Mohm DC feedback resistor issue ?? you said you guys recommended later on, is this recorded on some application notes ?? Plus I'm building a medical device which means I can not induce DC current into the user hence I can not use a 10k resistor and capacitor in parallel. can you please give me some more information on this, may be instead of using a 10M we can use a smaller resistor (1 to 5 M) to help with the DC feedback. thanks :)