Strange Optical Signal Noise with ADPD107

Hi FormerMember, I am currently using the ADPD107 for PPG sensing. I am getting some strange looking noise on the optical signal inconsistently. It seems to be more of a problem when sampling at lower LED brightnesses, but it is not always consistently noisy.This is what it looks like when it is really bad:

and sometimes it looks like this, with less frequent spikes:

Aside from electrical issues with the boards (which we are investigating), is this anything you have seen before and can diagnose? We are using a setup with 3 LEDs and 2 photodiodes. I believe I have optimized the timing correctly. Here is a dump of all the registers, perhaps are settings are causing this:

Register (Hex) Value (hex)
0 8000
1 FF
2 204
6 700
8 516
9 C8
A 0
B 1
D 0
F 0
10 2
11 1131
12 50
14 55D
15 0
18 2000
19 2000
1A 2000
1B 2000
1E 2000
1F 2000
20 2000
21 2000
22 1004
23 300C
24 3001
25 F1C
30 320
31 818
34 0
35 320
36 818
37 0
38 0
39 22D4
3B 22D4
3C 3006
42 1C37
43 ADA5
44 1C37
45 ADA5
4B 2692
4D 98
4E 60
4F 2090
50 0
54 20
55 0
58 0
5A 0
5F 0
  • I also want to add a couple more plots of the noise amplitude vs drive current for different LEDs:

    Both red and IR seem to have increased noise below 200 mA. I am wondering if this has something to do with the LED voltages. We are driving them with 5.5v, here are the forward voltage specs for the LEDs we are using:

    Low currents may create a scenario where LEDXx to LGND may be too high. Would this explain the behavior I am seeing?

  • +1
    •  Analog Employees 
    on Sep 2, 2017 4:22 PM

    Hello,

    There are three things that quickly come to mind that could possibly be causing your issues. 

     1. The driver pins are only guaranteed to 3.6V so you are certainly violating this spec by driving red and IR LEDs off of 5.5V. Even if you're at a high enough LED drive during the pulse such that the drop across the LED will be big enough to bring you back within the specified range of the pin, while the LED driver is "off" between pulses you will certainly be pulling the drive pin above 3.6V. The first thing you need to do is adjust your VLED supply such that the 3.6V LED driver pin spec is never violated.

    2. It is possible that there are disturbances on the LED supply or the supply to the ADPD105 that are causing increases in noise. The PSRR of the AVDD supply to the device is ~30dB so an LDO must be used to supply the ADPD105. Furthermore, we have seen that a noisy supply to the LED can cause disturbances to the LED drive current which would affect the signal received by the PD. What are you currently using to supply the LED? You could try to switch it with a known "clean" supply to see if it helps the problem.

    3. It is possible that high frequency ambient interferers are coupling into the PD, for example flourescent lights with high frequency switching ballasts. Is the setup arranged such that ambient interference sources can couple into the PD?

    I would start with #1 above because we certainly can't guarantee any performance of the device when the voltage levels applied to the pins exceed the maximum specs.

    Regards,

    Kevin

  • Thanks for the info, will fix the LED supply voltage. It appears that the root cause of the noise was from a noisy boost regulator supplying the LED voltage, switching to an LDO solved it.