This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

AFE4400SPO2EVM: How do I interpret the LED register value?

Part Number: AFE4400SPO2EVM

I'm using the AFE4400SPO2EVM UI tool to capture LED values. Reading the register value of 0x1FFF19, how do I translate it to Volts/Codes as shown in the ADC Capture tab? I see that the register value is in Hex, and is LSB first - that leaves me with a decimal value of 1703711 while the ADC capture page shows a range of ~100,000 in Code and 0.05 in volts.

  • Hello Vidya,

    Though the register values in the EVM are sent LSB first, the GUI arranges the bytes in order.
    So the register value of 0x1FFF19 correspond to decimal value 2096921. This corresponds to a saturation code.

    When the input exceeds the full-scale levels, the output code saturates. The exact saturation value depends on
    the NUMAV setting. For different NUMAV settings, the saturation value on the positive side is between 0x1FFF19
    and 0x1FFFFF.
  • Hi Praveen,

    I'm afraid I don't completely follow. It sounds like the LED register decimal value is mapped to a code using the saturation? Where can I find more information about the saturation code and NUMAV setting?
    Can I obtain the voltage level read by the photodiode directly?
    Thanks in advance!
  • Hi,

    0x1FFF19 is 2096921 in decimal. So to translate it into the voltage , you have to multiply it with one LSB as given below.

    2096921*1.2/(2^21) = 1.2V and since 1.2V is the reference voltage of the ADC, the ADC is said to be saturated with the given input.

    As said by Praveen the saturation on positive side is between 0x1FFF19 and 0x1FFFFF and the saturation value on the negative side is between 0xE080E7 and 0xE00000.

    Photodiode gives current (not voltage) proportional to the light it receives. The only way to read this is to pass it through the TIA to convert it into voltage and digitize it using the ADC before reading it out using SPI.

    Regards,
    Prabin
  • Thank you for the response Prabin. I will use that equation to calculate the voltage and report back here.

  • Editing to say I had to experiment with a few captures to understand it and now I'm able to calculate the voltage values. Thanks!