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AFE4300: Issues in impedance measurement using custom PCB

Augusto Mura
Prodigy 20 points
Part Number: AFE4300

Good morning,

I'm Augusto, I'm currently working on a project regarding leaf impedance measurements. Cc-ed readers are my colleague and Scientific Director of the project respectively. 
We ran into some issues using the AFE4300 ADC: we could not read a proper measurement. The only results we read so far are 0x0 and 0xFFFF (corresponding to the full scale range), despite using different loads, both resistive and capacitive. Also, we are especially interested in using QI demodulation settings.
List of tests that we carried out:
  1. Configuration register: it works properly. We're able to write on a register and read the input value.
  2. Measure AVDD via BAT_MON_EN: we measured it correctly.
  3. Check frequency of the exciting current (via external scope): it matches the value set via register configuration.
  4. Sensing, both with IQ demodulator and full wave rectifier: wrong measure. Always on 0x0 or 0xFFFF. We also tested differential and single ended configuration for both modalities.
  5. Tested different sample rates.
  6. Tried different frequencies for the IQ demodulator clock.
We bought evaluation board, and used it as explained in sbau201a application note. Everything went fine.
Given the results above, we think that we may have made some mistake in PCB design. Would it be possible for you to provide us the evaluation board schematic? Otherwise, also giving a look at the schematic of our PCB (please find it attached to this mail) would be helpful for us. If we forgot some information, or you have any questions about our design, don't hesitate to ask it.
Thank you in advance for your support.
Best regards,6136.schematic.pdf
  • 0
    TI__Mastermind 49716 points

    Hi Augusto,

    You can refer to the afe4300 evm schematics at the end of the EVM user guide at

  • 0 in reply to Praveen Aroul

    Hi Praveen,

    I am a colleague of Augusto.

    Thank you for your reply. We double-checked our schematic, and compared it to the available EVM schematic. We noticed that there are two main differences, regarding pins 72, 73, 74 and 75. indeed we left them not connected, as user guide says (page 38). On the other hand on the very same page, those pins are connected to AVDD_DUT, SIMO_M, AGND1 and AGND1 again, respectively. I was wondering if they are mandatory ignals, or if they must be left not connected.

    Also pins 7 and 8 are not connected in our design, because they are related only to weight-scale measurement, that we're not using. Are they needed even for the BCM measurement chain?

    Last, in calibration circuit (page 42) is it necessary to use reference resistor, connected respectively on pins RP1,RP0 and RN1,RN0, for the correct functioning of the AFE4300?

    Thank you in advance for your support.