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AFE4490: LED current DAC error

mingliang zhu
Prodigy 40 points
Part Number: AFE4490

Dear Sir:

     I'm testing the LED current DAC error.But it is different from the datasheet Electrical Characteristics(page11:  LED current DAC error ±5%). When I set the current 50mA,This  is my  test data below.

Module  test current
0 57mA
1 48mA
2 39mA
3 46mA
4 59mA
5 38mA

For  this  problem,i have asked your fae,he replied to me:" I got access to the AFE4490 production test data.
The min/max limits for the LED currents are set to +/-25% around the typical value.
From the customer results, looks like they are seeing a +/-20% spread across devices.
So their result seems to be within expected part-to-part variation."

Anyone knows why  is it  different from the datasheet?

Thanks!

   

  • 0
    TI__Genius 12555 points
    Hi Mingliang,

    Let me check with the team and get back by end of this week.

    Regards,
    Prabin
  • 0 in reply to Prabin Yadav
    TI__Intellectual 2885 points
    Hello Mingliang,

    I will be closing this thread due to inactivity, if you have further questions feel free to ask a new question. Thanks
  • 0 in reply to Prabin Yadav
    Prodigy 40 points
    Hi Prabin,
    Thank you for your reply and I'm looking forward to your reply again.

    Regards,
    Mingliang zhu
  • 0 in reply to mingliang zhu
    TI__Genius 12555 points
    Hi Mingliang ,

    I looked at the data and found that indeed it is 25%. The number in the datasheet is a typical number.
    That being said, can you please share your concern about the error in DAC code.
    As far as I know , the absolute accuracy of the DAC will have a less significance given that there will be variance in the LED's intensity across multiple LEDs for a given DAC curent.
    If you share some information about the concern that you have regarding the DAC's accuracy from performance point of view, we can work to find a solution to address those.

    Regards,
    Prabin
  • 0 in reply to Prabin Yadav
    Prodigy 40 points
    Hi Prabin,

    Thank you for your reply.For my application,I find that improve the LED current can improve Signal to noise ratio.Relative to the gain and sampling resistor, the effect of increasing the current is more pronounced,so I improve the led current in the case of meeting the requirements of led.But led specification say that the maximum led current is 100mA。When I set led current 100mA，it may be 125mA，it is bigger than led specification.

    Regards,
    Mingliang zhu
  • 0 in reply to mingliang zhu
    TI__Prodigy 215 points

    Hi,

      Normally there is a variation (spread) in the LED brightness and PD sensitivity as well. These variations, along with the LED current inaccuracy would cause a spread in the overall current transfer ratio (PD signal current/ LED current). So if you are interested in achieving a tighter distribution of the current transfer ratio across systems, maybe one option is to digitally adjust the LED current setting code based on the overall error. For example, if you had a system with an AFE that had a higher LED full scale current or brighter LED, you could scale down the LED current code to compensate the effect. 

    Regards,

    Anand.