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AFE4400 sampling rate

Chiheon Kwon
Prodigy 90 points
Other Parts Discussed in Thread: AFE4400

Hi, I am using AFE4400-MSP430 configuration to get PPG signal.

What I am trying to do is get PPG signal in a maximum sampling rate. 

So I looked up the maximum sampling rate in AFE4400 datasheet, which is 5000hz(PRP count is 800)

However, when I changed PRP count into 799 which was  set in default of 7999(500hz)  it starts not working indicating 0 volts.

After, I made PRP count 999(4KHZ), it starts working but output range suddenly narrowed into +- 0.4v, however much light fall onto diode, maximum output is just 0.4V, where I think it should be 1.2V(no cancellation current applied).

Then, I evaluated the output  by increasing PRP count step-wisely from 799 to 999, 1999, 3999.

Then I found when PRP count is 1999, output range becomes +-0.8v, and when PRP count is 3999, output range turns back into normal range +-1.2V.

Of course, I checked the output when PRP is 7999 and it works so well.

I have no idea why this phenomenon is happening.

I looked up datasheet again and again, but could't find any mention on relationship between sampling rate and ADC output range. 

Help me out.

Thank you.

-Jay

  • 0
    TI__Mastermind 49956 points
    There is a typo in the datasheet. Control register 1 (CONTROL1) bit 1 must be set to 0 but in the datasheet it is mentioned that bit 1 must be set to 1.
    We are fixing this in the next revision of the AFE4400 datasheet.
    This should resolve the output range for higher PRP counts.
  • 0 in reply to Praveen Aroul
    Prodigy 90 points
    Thanks a lot.
    Problems have been fixed with change register of CONTROL1, which was I struggled with for a few days, though.

    And one more related question.
    I want to make sampling rate at 5000hz, which I calculate PRP count should be 800.
    If I divide PRP 800 equally by 4(4 phases), then maximum time which will be used to conversion phase is only 200 count which is 50us.
    In the datasheet, they mention ADC takes 50us to convert one sample.
    Conversion phase has to take place after ADC reset on/off occurs which is I set as 3 clocks
    As such, the time I can set is less than 200 count (at most 196 count) which means less than 50us.
    Is is okay to set adc conversion phase interval less than 200 count(50us)?

    Also, above timing problem applies to sample phase interval.
    I see in the datasheet that sample phase starts after LED On time minus settling time which I don't know exactly.
    So if I have to take into account settling time when I am setting sample phase count start register, the maximum time I have for sample phase is less than 50 us like conversion time problem above.
    however, datasheet mentions Rx sample time needs at least 50us.

    I appreciate your helping.

    -Jay
  • 0 in reply to Chiheon Kwon
    Prodigy 90 points
    It is not working at all at sample rate of 5000hz.
    I checked it works well at sample rate up to 4000hz.
  • 0 in reply to Chiheon Kwon
    TI__Mastermind 49956 points

    Hello Jay,

    Limit the maximum PRPCOUNT to 900 to achieve a PRF of 4.44 kHz.

    The recommendation to have a wait time between the start of the LED pulse and the start of the sampling phase is to allow margin for the TIA output to settle before it gets connected to the filter.