Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

  • State TI Thinks Resolved
  • Replies 2 replies
  • Answers 1 answer
  • Subscribers 46 subscribers
  • Views 41 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

AFE4404: Minimum LEDs duty cycle and time to sampling

Michael Pasternak
Prodigy 60 points
Part Number: AFE4404

Tool/software:

Dear Sirs.

I use AFE4404 in low power device. I need to reduce power consumption of LEDs.

Pulse repetition factor is 10ms

Today I switch LED on for 230us and wait 70us between switching the LED On and starting sampling.

It works properly but power consumption is too high.

What is a minimum time that I can switch LEDs ON?

What is a minimum time between switching the LED on and starting sampling? Capacity of photodiode less than 40 pF

Table 7 in datasheet is not clear.

Please advice.

  • 0
    TI__Guru 63126 points

    Hi Michael,

    Thank you for your post.

    Theoretically, the minimum LED pulse duration would be one CLK_TE period, but this would likely be impractical for your application. CLK_TE is the Timing Engine clock reference, which is equal to the ADC clock divided by the CLKDIV_PRF setting.

    The minimum time between the LED turning on and the start of sampling is defined as t1 in Table 7. This minimum time is typically 20% of the LED on time, but cannot be any less than 25 µs. For low-power applications, we generally recommend starting with an LED pulse duration of 50-100 µs and using a higher TIA gain setting.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 60 points

    Thank you for fast response.

    But I am still confused about sampling process.

    How does sampling work? As I see in block diagram Figure 23, sample time charge capacitor CSAMPx. If sample time is enough to charge this capacitor, sampling time more than this shall not affect of DC value. What is a time that enough to charge it to for example 99%?

    Example:

    Assume sampling time is 200 us the same LEDs, LEDs currents,  photodiode and the same optical; pass.

    Assume I get from ADC DC signal 500000 ADC units.

    What will happen with signal if I only change sample time? For example reduce sample time from 200 us to 100 us?

    Will I get from ADC the same 500000 or something else?

    Please advise.