Author Topic: Photodiodes saturating in ambient light  (Read 7224 times)

0 Members and 1 Guest are viewing this topic.

Offline aussie_laser_dudeTopic starter

  • Contributor
  • Posts: 46
  • Country: au
Photodiodes saturating in ambient light
« on: May 10, 2020, 11:37:39 am »
Hi guys! I've made a photodiode array that works in the shade but not in the sun.

The photodiodes are ~2mm silicon photoconductive sensors. The goal is to measure brief light source pulses of 1mW power lasting 10 microseconds.
  I've connected in a 3.3V power supply to a 13komhm resistor in series with the photodiodes (when light hits the sensors, the resistance falls from 15kohm to low ohms and voltage across the series resistors goes high, which i measure on a scope).
  The oscilloscope shows beautiful curves, everything is perfect! But then i try it in sunlight... The signal is too small to measure. Hmmm i'm a bit unsure how to proceed?

Here's my guesses:
1. Optically filter out the ambient sunlight using a bandpass filter

2. Use a 1 nF capacitor to remove ambient signal since it's dc. Attach an amplifier of some sort to each photodiode in the array. I have no experience at this? FET? Op'amps?

3. Don't apply an external voltage to the sensors. Let them generate their own voltage when light hits them. Amplify the voltage somehow?

To explain the issue another way, the sensors change in resistance is easily measurable in the shade, going from high to low kohm when struck by pulsed light. When in sunlight it's resistance drops to nearly complete zero, <0.01 ohms. Any pulsed light added on top of the sunlight causes only a tiny change in resistance that i can't measure on my scope.

Any ideas appreciated.
« Last Edit: May 10, 2020, 12:09:50 pm by aussie_laser_dude »
 

Offline KMoffett

  • Regular Contributor
  • *
  • Posts: 96
  • Country: us
Re: Photodiodes saturating in ambient light
« Reply #1 on: May 10, 2020, 11:50:01 am »
Is the pulsed light source and the photo detector in fixed positions?
 
The following users thanked this post: aussie_laser_dude

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 10174
  • Country: gb
Re: Photodiodes saturating in ambient light
« Reply #2 on: May 10, 2020, 12:17:30 pm »
Optical filtering would probably be the most effective - that would be tackling the problem at source rather than trying to sticking plaster around it. It's also the approach taken by standard IR receivers (that and AGC).
Best Regards, Chris
 
The following users thanked this post: aussie_laser_dude

Offline aussie_laser_dudeTopic starter

  • Contributor
  • Posts: 46
  • Country: au
Re: Photodiodes saturating in ambient light
« Reply #3 on: May 10, 2020, 12:19:59 pm »
They'll be moving all over the place. Ambient light hitting the detector will change constantly as the detector is moved in and out of shadows and turned around etc. A simple 1nF capacitor does a nice job fixing the dc offset to 0 volts, but the signal strength of the pulsed light in sunlight is horrible. Look at the photos i've attached, there's an oscilloscope curve measured in an "indoors ambient light environment" which is perfect, outside in the sun it's a very flat line, no noticeable voltage peaks.
 

Offline aussie_laser_dudeTopic starter

  • Contributor
  • Posts: 46
  • Country: au
Re: Photodiodes saturating in ambient light
« Reply #4 on: May 10, 2020, 12:44:00 pm »
Thanks Gyro, yes optical filter sounds like the best thing, i've ordered a $100 optical bandpass filter (ouch, this approach isn't cheap). I'd like to be able to understand the electronic approaches too, i like learning this stuff :D. A 50nm bandpass filter will make it so the ambient light and pulsed light have similar power levels, so there'll still be a reduction in amplitude of the peaks, but just being able to measure at all would be awesome!
 I'm looking into the Automatic Gain control you suggested, that's kinda what i'm after. I'm not good enough at EE to know if a simple FET /opamp would fix everything?
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 22436
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Photodiodes saturating in ambient light
« Reply #5 on: May 10, 2020, 12:53:17 pm »
Well yeah, sunlight is comparable irradiance to that puny laser.

If it's a red laser, note that filtering only gets you so much.  Or IR.  Plenty of both in sunlight.

One defining characteristic of your problem would seem to be the pulse width.  Perhaps the laser diodes can be AC coupled, i.e. supplied by an inductance from a CV supply, and the signal coupled out with a capacitor, removing DC offset.  (The inductor would most likely be emulated with a transistor gyrator circuit, giving a modest and controllable impedance, even to low frequencies if needed, and implementing current limiting protection for instance.)  Any correlation between signals (e.g., one pulse is detected so another pulse follows on the next, and so on) would be valuable information, and can be incorporated into the detection circuit (or analyzed wholesale by digitizing everything and doing statistics on a PC or whatever).

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
The following users thanked this post: aussie_laser_dude

Offline tggzzz

  • Super Contributor
  • ***
  • Posts: 21227
  • Country: gb
  • Numbers, not adjectives
    • Having fun doing more, with less
Re: Photodiodes saturating in ambient light
« Reply #6 on: May 10, 2020, 01:02:51 pm »
Firstly show us a schematic; there's little point in us playing "20 questions" trying to figure out how we might help.

Secondly don't think in terms of a photodiode's resistance. Do think in terms of the current. That will help you understand what you can do to minimise the problem.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 
The following users thanked this post: Yansi, aussie_laser_dude

Offline David Hess

  • Super Contributor
  • ***
  • Posts: 17428
  • Country: us
  • DavidH
Re: Photodiodes saturating in ambient light
« Reply #7 on: May 10, 2020, 01:40:14 pm »
AC coupling the photodiode amplifier to remove ambient light is the common solution.  The best way to do this is with a DC restoration loop around the photodiode amplifier.
 
The following users thanked this post: aussie_laser_dude

Offline Siwastaja

  • Super Contributor
  • ***
  • Posts: 9336
  • Country: fi
Re: Photodiodes saturating in ambient light
« Reply #8 on: May 10, 2020, 02:00:13 pm »
Note that sunlight has a smooth spectrum (some minor spiking is there, but irrelevant here).

Laser, OTOH, is extremely narrow bandwidth, and very repeatable i.e. the band does not drift.

Hence, using tight optical bandpass filters, thing about FWHM=10..20nm or so, is an easy way to decrease the sunlight contribution by maybe two orders of magnitude.

It isn't very low-cost, though; proper filters cost some serious money, they are of dichroic (interference) type. First try to deal with it electrically, which is OK with the following limitations:
* Sunlight contribution needs to be in the linear range of the sensor to be removed (if the sensor itself saturates, there is no difference if you hit it with more light, nothing AC coupling can help),
* Shot noise in the sunlight is passed through the DC removal. The more light there is, the more noise.

If you can't achieve good enough results electronically, then the next step is consider optical filtering, additionally.

In any case, we need the part number for the photodiode (or a very similar one if you can't disclose the actual part), and a schematic of your current sensing circuitry.
 
The following users thanked this post: aussie_laser_dude

Offline aussie_laser_dudeTopic starter

  • Contributor
  • Posts: 46
  • Country: au
Re: Photodiodes saturating in ambient light
« Reply #9 on: May 10, 2020, 02:28:29 pm »
Thanks guys,
I've attached a circuit diagram for a single diode (photodiode is BPW34S high speed silicon PIN photodiode).
Thanks Tim and David, I've tried ac coupling via a simple 1nf capacitor, it's working well to remove the slow changing offset caused by ambient light. The fast changing signal is all that's wanted here. Unfortunately the signal amplitude drops to near zero with ambient sunlight :( a dc restoration loop? I'll investigate that.
  Thanks tggzzz, i'm trying to solve in terms of current, doing the math is proving challenging. Intuitively i expect either
a) the photodiode actually saturates from too much sunlight and the current is a flat dc curve even with the extra pulsed light. OR
b) it doesn't actually saturate, there's a big dc current with tiny periodic spikes from the pulsed light. The capacitor in my circuit should remove the dc offset and keep the spikes... I don't see the ac spikes when measuring with ambient sunlight so i guess that means it's case a, the photodiode can't create anymore free electrons? ie. It's optically saturated!?
« Last Edit: May 10, 2020, 02:58:59 pm by aussie_laser_dude »
 

Online ejeffrey

  • Super Contributor
  • ***
  • Posts: 4034
  • Country: us
Re: Photodiodes saturating in ambient light
« Reply #10 on: May 10, 2020, 03:08:09 pm »
Increase your bias voltage or decrease the shunt resistor.  You aren't saturating the photodiode you are saturating the battery+resistor.  Drop the resistor to 1k and add an extra gain stage and should be fine.
 
The following users thanked this post: aussie_laser_dude

Online ejeffrey

  • Super Contributor
  • ***
  • Posts: 4034
  • Country: us
Re: Photodiodes saturating in ambient light
« Reply #11 on: May 10, 2020, 03:26:10 pm »
Also, always draw a photodiode as a diode not just a circle.
 
The following users thanked this post: aussie_laser_dude

Online ejeffrey

  • Super Contributor
  • ***
  • Posts: 4034
  • Country: us
Re: Photodiodes saturating in ambient light
« Reply #12 on: May 10, 2020, 03:30:23 pm »
Wait: is your sensor a photodiode reverse biased (ie in photoconductive mode) or a photoconductive sensor or LDR?
 
The following users thanked this post: aussie_laser_dude

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 9003
  • Country: us
  • Retired, now restoring antique test equipment
Re: Photodiodes saturating in ambient light
« Reply #13 on: May 10, 2020, 03:31:24 pm »
Your circuit will "saturate" when the DC photocurrent through the 13k resistor produces a voltage that overcomes your bias battery voltage and the parasitic diode of the photodiode forward biases, shorting out the photocurrent and preventing it from reaching your output meter.
 
The following users thanked this post: aussie_laser_dude

Offline tggzzz

  • Super Contributor
  • ***
  • Posts: 21227
  • Country: gb
  • Numbers, not adjectives
    • Having fun doing more, with less
Re: Photodiodes saturating in ambient light
« Reply #14 on: May 10, 2020, 03:38:16 pm »
Thanks guys,
I've attached a circuit diagram for a single diode (photodiode is BPW34S high speed silicon PIN photodiode).
Thanks Tim and David, I've tried ac coupling via a simple 1nf capacitor, it's working well to remove the slow changing offset caused by ambient light. The fast changing signal is all that's wanted here. Unfortunately the signal amplitude drops to near zero with ambient sunlight :( a dc restoration loop? I'll investigate that.
  Thanks tggzzz, i'm trying to solve in terms of current, doing the math is proving challenging. Intuitively i expect either
a) the photodiode actually saturates from too much sunlight and the current is a flat dc curve even with the extra pulsed light. OR
b) it doesn't actually saturate, there's a big dc current with tiny periodic spikes from the pulsed light. The capacitor in my circuit should remove the dc offset and keep the spikes... I don't see the ac spikes when measuring with ambient sunlight so i guess that means it's case a, the photodiode can't create anymore free electrons? ie. It's optically saturated!?

That's half a circuit diagram: it doesn't indicate which way round the photodiode is connected. Yes, it matters!

Assuming it is reverse biassed, then the arithmetic is simple. If you have x watts of light falling on the PD then it will have a current of i=x/2 amps. The voltage drop across the resistor will then be 13000*i. If we assume that we want >1V reverse bias across the PD, then the max voltage across the resistor can be 2.3V, and from that you can work out the maximum allowable light before saturation.

You should also do research on opamp transimpedance amplifiers. That will help you improve the sensitivity and frequency response.

You would also benefit from removing the 1nF cap in series with the resistor (why aren't you using AC coupling?). Then monitor the voltage while the 10us light pulses are there. Then gradually increase the ambient light (torch?) and observe the voltage. That should help you understand what is happening.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 
The following users thanked this post: aussie_laser_dude

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 22436
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Photodiodes saturating in ambient light
« Reply #15 on: May 10, 2020, 06:15:17 pm »
That's why I said use an inductance, or something emulating it -- the resistor allows the photodiode to saturate and the gain drops towards zero.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
The following users thanked this post: aussie_laser_dude

Online twospoons

  • Frequent Contributor
  • **
  • Posts: 270
  • Country: nz
Re: Photodiodes saturating in ambient light
« Reply #16 on: May 10, 2020, 11:04:09 pm »
In a previous job I built an IR laser receiver using BPW34s that could detect a 1pJ pulse in full sun. Your pulse is ~10nJ, so it should be fairly easy. The circuit used the BPW34s in photovoltaic mode (no bias) and fed the photocurrent through a small pulse transformer to remove the DC current produced by ambient light. There was a fairly high gain pulse amplifier after that, built using BFT92 BJTs.

If you dont want to use photovoltaic mode, which can be a bit slower, then you might want to try using a gyrator as the bias load (one BJT, one cap, one resistor).
Both methods shown below, but ignore the component values - they're just the schematic editor defaults.

« Last Edit: May 10, 2020, 11:13:03 pm by twospoons »
 
The following users thanked this post: aussie_laser_dude

Offline David Hess

  • Super Contributor
  • ***
  • Posts: 17428
  • Country: us
  • DavidH
Re: Photodiodes saturating in ambient light
« Reply #17 on: May 11, 2020, 09:22:22 am »
a dc restoration loop? I'll investigate that.

If the photodiode amplifier is inverting, then an integrator which is also inverting can compare the output to zero and subtract a current from the input to stabilize the output.  This produces a high pass response from the integrator's low pass response.

https://e2e.ti.com/support/amplifiers/f/14/t/300533?AC-Coupled-Photodiode

This integrator trick also works with instrumentation amplifiers to produce a high pass response.
 
The following users thanked this post: aussie_laser_dude

Offline aussie_laser_dudeTopic starter

  • Contributor
  • Posts: 46
  • Country: au
Re: Photodiodes saturating in ambient light
« Reply #18 on: May 11, 2020, 10:02:04 pm »
Thanks for the messages guys, lots of info to take in. Sorry i messed up my terminology, i've used reverse bias mode and also photovoltaic mode (not photoconductive). Both circuits have similar outputs in sunlight, but the reverse bias circuit has a much stronger signal in the shade. Should fully understand the battery/resistor saturation theory later today after a few more tests, also nearly got a bjt pnp amplifier ready to test and op amps are in the mail so this is exciting!

Twospoons, was the IR wavelength of your device above 800nm? It's got quite impressive specs for being bathed in sunlight, i'm guessing the design had an optical low pass / bandpass filter?
 

Offline StillTrying

  • Super Contributor
  • ***
  • Posts: 2850
  • Country: se
  • Country: Broken Britain
Re: Photodiodes saturating in ambient light
« Reply #19 on: May 11, 2020, 10:57:40 pm »
All you've got to do to prevent saturating is to lower the 13k to about 2k2.

Start simple to find out what the photo diode's output current is.
https://www.eevblog.com/forum/beginners/more-help-with-ir-photo-diodes-please/msg3035868/#msg3035868
.  That took much longer than I thought it would.
 
The following users thanked this post: aussie_laser_dude

Online twospoons

  • Frequent Contributor
  • **
  • Posts: 270
  • Country: nz
Re: Photodiodes saturating in ambient light
« Reply #20 on: May 12, 2020, 01:35:14 am »
It was around the 800nm wavelength. This was a while ago so some details are a little hazy . The version of BPW34 we were using had a built-in longpass daylight filter. Just a dye incorporated in the package.
Running reverse bias does usually yield more signal - we were using photovoltaic mode to save power in a battery operated system. We would have needed to supply over 30mA otherwise (lots of photodiodes).

Attached is one trick to getting good signal in photovoltaic mode.  The transformer takes care of the DC current, and the transistor acts as a trans-impedance amplifier.
 
The following users thanked this post: aussie_laser_dude

Offline aussie_laser_dudeTopic starter

  • Contributor
  • Posts: 46
  • Country: au
Re: Photodiodes saturating in ambient light
« Reply #21 on: May 12, 2020, 02:17:19 am »
Quote
The version of BPW34 we were using had a built-in longpass daylight filter.

ahhh haha, numbers make a lot more sense to me now.

Thanks guys, figured out most of the details, using a 1 kohm resistor in reverse bias mode does actually work quite well. The signal is very low (~5mV100mV), definitely needs amplification, but now the AC signal remains a constant amplitude when the ambient sunlight changes. I've attached a pic of it operating in sunlight nicely and also a pic of my photodiode math/theory. Thanks tggzzz, ejeffrey, TimFox, T3sl4co1l & StillTrying for help on understanding this stuff :)

Sweet, that's half the problem solved!

 Now onto the amplification side of things, pretty hyped to try out some different amplification methods, I do also plan to save battery using the photovoltaic approach (but happy to try out reverse bias circuits in early prototypes, whatever gets the prototype working), thanks twospoons, will definitely look into the circuits you've given.

Thanks once again guys, one step closer turning an idea into reality  :-+
« Last Edit: May 14, 2020, 06:32:34 am by aussie_laser_dude »
 

Offline aussie_laser_dudeTopic starter

  • Contributor
  • Posts: 46
  • Country: au
Re: Photodiodes saturating in ambient light
« Reply #22 on: May 12, 2020, 10:21:46 am »
Could I get some help with understanding twospoons small pulse transformer photovoltaic circuit in "reply 16". Transformers scare me, I don't understand transformer details very well.

My understanding is the transformer will have a limited bandwidth, I have no idea how to calculate transformer properties and load resistor R1 that will give a bandwidth of say >3 MHz for the photovoltaic photodiode.

Could R1 be changed from 1 Kohm to 10 Kohm without ruining the circuit bandwidth? My reasoning is that the large DC current from the photodiode won't be passing through the R1 resistor, so we can use a higher value resistor to get a higher potential difference. I'm worried this will sacrifice the bandwidth of the transformer or photovoltaic photodiode, but I'm too dumb to understand how to figure this out. I have no idea of the math. If anyone has a good link that explains these properties of transformers I'd love to have a look.

Also, could a 1:4 transformer be used, ie. 1mV AC in and 4mV ac out on other side? Would a transformer like this ruin the bandwidth somehow? Is it better to stick to 1:1 transformers?

Thanks for any help
« Last Edit: May 12, 2020, 10:23:44 am by aussie_laser_dude »
 

Offline SparkyFX

  • Frequent Contributor
  • **
  • Posts: 676
  • Country: de
Re: Photodiodes saturating in ambient light
« Reply #23 on: May 12, 2020, 12:44:38 pm »
You could try a polarizing filter in front of the receiver, those filter out ambient stray light very efficient. I can't tell if all lasers are polarized in general or you need to arrange the laser in a specific orientation, could you work with that?
Support your local planet.
 
The following users thanked this post: aussie_laser_dude

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 9003
  • Country: us
  • Retired, now restoring antique test equipment
Re: Photodiodes saturating in ambient light
« Reply #24 on: May 12, 2020, 01:14:44 pm »
Be careful with Polaroid-type (absorptive) polarizing filters and lasers.  As a young graduate student, I burned a hole in a Polaroid by sending a low-power laser beam through it:  the polarizer works by absorbing the power with the wrong polarization.
 
The following users thanked this post: aussie_laser_dude


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf