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ADC_RDY signal goes high continuously even when the PulseOx sensor cable not connected to AFE44x0SPO2EVM

Anil Shetty
Prodigy 70 points
Other Parts Discussed in Thread: AFE4400

Hi,

We are developing HRM based on AFE4400 on Android. We are trying to validate it on AFE44x0SPO2EVM board. We have set the PRF to 400Hz with a duty cycle of 25%.

We are observing that after software reset, the read to these registers returns 0, which is as expected. But after configuring the control and timing registers, we are always getting ADC_RDY interrupt on the host processor side. Further testing revealed that the LED and Ambient LED digital sample value registers are getting updated even when the PULSEOx Cable is not connected to the DB9-F connector.

We are confused as to how the LED and ALED digital sample registers are getting updated without connecting the finger sensor pulse oximeter cable.

Could you please advise us on this?

Thank You.

  • 0
    TI__Mastermind 49956 points

    Hello Anil,

    In AFE4400, the ADC gets updated every PRF irrespective of the sensor cable connected to the AFE after configuring the control and timing registers.

    To verify the status of the sensor cable, the MCU should be programmed to run a diagnostic mode check periodically. The AFE does not run the diagnostic check automatically.

  • 0 in reply to Praveen Aroul
    Prodigy 70 points
    Hi Praveen,

    Thank you very much for the reply.

    As suggested we ran the diagnostic mode and it completed successfully without setting any fault flags in the DIAG resistor.

    We are now observing that the Ambient LED register values are getting updated irrespective of finger sensor pulse ox cable connected or not. The LED register values are fixed at 0x1FFFFF whether finger sensor cable is connected or not. According to the datasheet, the above value is the 22 bit ADC output code for differential voltage input of 1.2V. We are confused as to how to interpret the ADC output code values we read from the LED and ALED registers.
    The LED and photo diode are not on the EVM board but in the finger sensor cable, but still the ALED register values are getting updated at every ADC_RDY signal.
    Could you please clear this confusion and advice us on how to interpret the ADC output code to calculate heart rate in beats per minute?

    Additional Info:
    We are not using Ambient cancellation and second stage gain. The value set in TIA_AMB_GAIN register is 0x06. The TIAGAIN register is used for factory use as mentioned in the AFE4400 datasheet. But after reset it will be 0x00h. Should we set some value or leave it as it is?

    Thanks,
    Anil
  • 0 in reply to Anil Shetty
    TI__Mastermind 18285 points
    Anil,

    This is a 22bit 2's compliment ADC output code. Therefore, I believe when you are reading 0x1FFFFF when the cable is connected that is correct, but when it is not connected you have the signed bit and it is negative.

    To interpret the ADC codes, please follow the FAQ section (question # 18) in the link below to get an understanding on how to get HR data from the ADC output.

    e2e.ti.com/.../369445
  • 0 in reply to Anil Shetty
    TI__Mastermind 18285 points
    Anil,

    No problem, happy to help. Please verify the post if your questions have been answered.