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AFE operation sequence

Ranjith Kumbazha Omanakuttanpillai
Prodigy 190 points
Other Parts Discussed in Thread: AFE4490

I'm planning to implement a pulse oxymeter system with AFE4490 and a microcontroller( or Arduino board)

For this I need better idea about flow of the sequence of operation of AFE4490 and time required for them.

Sequence of operations & communications, that I understood from AFE4490 datasheet is as follows

1)      First we have to initialize the power up sequence for   AFE4490 (ie have to supply Reset pulse and to load the register values  from the controller). These tasks take a few Seconds to settle and it is a onetime process on starting of device operation.

2)      Then  In a pulse repetition period (PRP)  time,  IRLED: IR ambient :RED LED : RED ambient: (IRLED- IR ambient) : (REDLED- RED ambient) data will be stored in six  24 bit registers in AFE4490 and then ADC Ready pin will become high. These operations will be completed in one PRP. Then controller can read each register contents sequentially.

3)      Then Micro controller calculates the ambient light error value & LED brightness error values.  (Also calculate the PI and HR values etc.)

4)      Corresponding correction settings based on ambient error value & LED brightness error values got in   step 3 will be set to corresponding registers in AFE4490 through SPI.

5)      the sequence of operation  the steps from 2-3-4 are repeats continuously.  

Is this sequence operation is correct? if not please

Another doubt is, am I need to set the ambient cancellation value & LED brightness value (above step 4) from controller to AFE in every sequence? or only initially or after a particular set of sequence ? Or microcontroller have to check it continuously and have to configure  AFE setting to correct it when exceeds the limit? 

  • 0
    TI__Mastermind 49686 points

    Hello Ranjith,

    You do not need to run steps 3 and 4 for every sequence.

    The ambient light error value & LED brightness error value can be estimated for a block of samples instead of every sample. After you have applied the settings to the AFE, you can wait for brief amount of time and repeat the sequence again.

    You can set a low (for example 10% ADC full scale range) and high threshold (for example 90% ADC full scale range) and you can run this algo when the ADC codes for the LED phase exceeds the high threshold or goes below the low threshold.

  • 0 in reply to Praveen Aroul
    Prodigy 190 points

    Hi Praveen

    Thanks for the info

    What I understood from your reply is

    I have to store a predefined (say 10)number of LED and ambient values from AFE in my micro controller memory and then average of the stored 10 samples values have to be calculated and the resultant  four values (ie RED LED, RED ambient, IR LED)  can be used for checking the algos . Then if the  values are not in suitable range, have to correct it otherwise can repeat the process. 

    is it right? 

    I think ADC algo you mentioned in reply  for checking the ambient DAC  correction,   right?

    then how can I calculate the LED brightness setting value?

    Is the last two AFE output  registers values ( RED ambient  subtracted from RED LED  & IR ambient subtracted from IR LED) is suitable for  comparing with the ADC threshold values in algo?

     

  • 0 in reply to Ranjith Kumbazha Omanakuttanpillai
    TI__Mastermind 49686 points

    Hello Ranjtih,

    Yes your understanding is correct.

    The general idea is to target the DC voltage at the ADC input to approximately 50% of its full scale.

    To achieve this, a combination of LED current (LED brightness) and/or TIA gain can be adjusted.

    Yes you can use the last two AFE output registers for the algo.