# Using a photodiode in near darkness

1. Feb 23, 2012

### Artlav

Hello.

How can get maximum sensitivity out of a photodiode?

So far the best result i got is by using a current-to-voltage amplifier:

But with R at 1 MOhm it's still not good enough, and any more causes the thing to start oscillating out of control.

Would adding another amplifier after that improve the situation, or would it just amplify some noise? If yes, what kind of amplifier should it be?

Is there a way or point to remove whatever noise or oscillation this circuit gives with R>1MOhm, and keep adding resistors?

Or, how to do it some other (proper?) way?

2. Feb 23, 2012

### dlgoff

According to the AD820 Data Sheet, for a single supply, the voltage can be +3 V to +36 V. You could try a higher supply voltage than the 5 V you're showing. i.e. if you're looking for a larger output voltage.

3. Feb 23, 2012

### yungman

Just by the fact that the Rf=1M tells me that your photo diode give a lot of current....in close to 1uA range. This is no where even push any boundary unless you are looking for 100MHz + bandwidth.

You are right into transimpedance amp that we had two post talk extensively about. Tell us what is the condition, frequency response requirement etc.

For oscillation, first you have to tell us what is the capacitance of the photo diode. It is all about adjusting the close loop poles and zeros to tame the amp.

Post the spec and we'll talk more. I have a suspicion that you have parasitic problem that has nothing to do with the circuit in the schematic. eg, if the lead from the detector to the amp is very long, you'll have a serious problem.

What happen, all of a sudden transimpedance amp is in style?!!!!

4. Feb 24, 2012

### Artlav

I should mention that i had no prior experience with op-amps, and all i do now is based on a few days worth of reading the theory and experimenting.

As far as i understand, supply voltage defines the range of the output, not amplification?
So, if i'm trying to tell millivolts apart, what good would it be?

Is it normal to have MOhm range resistance in this kind of application?
I.e. is there a limit on how low a current can be amplified that way?

Light level measurement, 10-100 samples per second ideal.
With the provided circuit i get output indistinguishable from zero most of the time.

Not specified, nothing to measure with.

The oscillations are white noise or close it it.
In case of 2M and 0.1uf it looks similar to 1M and no capacitor.
With 1M and 0.1uf there is no detectable noise.

Lead length is supposed to be half a meter, but the problem appears with the diode sitting right next to the amp.

5. Feb 24, 2012

### skeptic2

Here is an application bulletin from Burr Brown, a manufacturer of very high quality op amps. You might try some of the ideas in it.

http://www.ti.com/lit/an/sboa061/sboa061.pdf

6. Feb 24, 2012

### yungman

How do you come up with the 1M as feedback resistor? What is the range of current you are trying to detect?

You put a 0.1uF cap across the 1M resistor, your frequency response is 1.59Hz!!! That is too slow. Transimpedance amp is more than just design basic op-amp circuit. Read the article Skeptic2 posted.

If your lead is 1/2m, you might have to put a 50Ω resistor in series with the photo diode close to the input of the op-amp to give some isolation.

7. Feb 24, 2012

### Artlav

Thanks, that's certainly of use.

Indeed it is in full one, and i was checking abbreviated one.
Diode Capacitance:
(VR = 0 V, f = 1 MHz, E = 0) 70 pF
(VR = 3 V, f = 1 MHz, E = 0) 40 pF

Also, i've drawn the diode wrong way around in the schematic above.

V=IR, and 1M is the largest resistor i've seen on sale.
So i guesstimeted it to be close to the limit or something.

Now, a Giga-Ohm resistor?
Not sure where to buy, not straightforward to make, nothing to measure with directly.
10M ones seem to be available, so about 100M is the highest i know i can go at the moment.

Apparently, below 5nA.
Assuming 5nA to be the upper level (at 5V), 20pA would be the lower sensitivity level.

If i understand correctly that it's 1/(2*pi*R*C), then for 100Hz i'll need C about 1.5nF?

I don't have anything from this range at hand today, so can't check.
The noise is what troubles me - with 2M it exist with any C i tried (100pF, 47nF, 100nF) and is white noise, while with 1M it only appear at 100pF and less and looks like mains hum with something about 4Hz added over.

I've looked over MT-050 article from the thread below, and the diode's capacitance seems to be the C1, R1 is the lead resistance, and C2 and R2 are out feedback/gain ones?
But i'm too slow at the moment to get just what does this give me, and how does the stark change in the noise kind come in.

That is certainly of use.
Also, this thread that you seem to have mentioned, is an interesting read.

Where does the number come from?

8. Feb 24, 2012

### yungman

Yes, you need to learn more as you have to taylor the circuit. Use the last op-amp suggest in that post.

9. Feb 24, 2012

### Artlav

Good news, i've figured out where the white noise came from - for the 2+ MΩ tests i put several 1MΩ resistors in series, but the capacitor spanned only the first one!

With that fixed, i got it up to 10MΩ, and things still look good.
Looks like finding these mythical 1GΩ resistors could actually be worth the trouble. :)

Thanks for all the info, that'll take some time to digest.

10. Feb 24, 2012

### yungman

Look up Caddock resistor. 1G is nothing!!! Make sure to get a resistor with longer body because the surface leakage might present problem if the body is too short. If you don't want to deal with 1G, do the 100M and follow with the next stage with gain of 10. Read the post you put in the last one, read the link in detail. You need exactly the gain as that one. All the discussion implies to you. You are lucky that you don't need high speed like the other post, for you, it would be much easier. The other poster is running into wall at the moment because his detector is higher capacitance, and he need much higher speed.

11. Feb 25, 2012

### Artlav

Noise is a problem.
On a breadboard with 50 MΩ (5x10M) and 10pF there is no detectable noise.

But for real, with almost a meter long leads there is noise.
Adding 100Ω in series near the amp didn't help.

In the dark the noise looks like this (FFT at the output, 0-100Hz):

Does this look familiar?
Any ideas how to fix it?

Also, i've never worked with resistance this high before - anything to be aware of? Like leakage across a moisture film along the board or something?

12. Feb 25, 2012

### skeptic2

With leads a meter long, are you using shielded cable? In the Burr Brown Bulletin, the oval loop tied to ground represents shielded cable.

13. Feb 25, 2012

### yungman

You are picking up noise on the long input wires. Look into what Skeptic2 suggested.

As for resistance, don't be afraid, it's just a resistor. I gave you the hint that you need long body resistor. When I did the transimpedance amp with high value resistor, I used DIP package and I bent the -ve input up and solder the resistor on the pin up in the air. You need very good cleaning spray, compressed air to blow away the solvent right after cleaning. 1G is not that high, you do this, you'll likely be fine.

If you measure down to pA, watch out for vibration. I worked on Mass Spectrometers and the vibration from the turbo pumps introduce vibration noise. We end up using rigid coax for input and mount the amp on the frame to lower the vibration to the amp.

14. Feb 25, 2012

### Artlav

The most annoying part is a ~2-4Hz regular-looking oscillation, with up to +-0.5V of amplitude at output. That one happens on a breadboard as well, with no extra wires.
It's not in the light or surroundings - happens from many lightsources and in many places, don't happen in none.
The "no detectable noise" part was about full darkness and near the upper limit.

Have i missed something again?

15. Feb 25, 2012

### yungman

This is unique in your situation. You have to chase down the 2 to 4 Hz noise.

Can you explain:

It's not in the light or surroundings - happens from many lightsources and in many places, don't happen in none.
The "no detectable noise" part was about full darkness and near the upper limit.

Is it a regular periodic frequency or random of about 2 to 4 Hz? If it is happening on the breadboard, then put Faraday cage around as see whether it is EM or not. Also tape the top of the photo diode so no light can go through and see whether that change the amplitude.

16. Feb 25, 2012

### Artlav

Regular and periodic, but not real - going from 100 samples per second to 200 turned it into a much higher frequency one, also periodic. Something is interfering or plain broken in the serial link part.

Time-out, i shouldn't be asking questions until i rule out all the obvious possibilities.

17. Feb 25, 2012

### yungman

Now you are talking. Good thing you can produce the problem on the breadboard, try different shielding method including blocking light, copper tapes and report back. Use a scope to look at the signal, don't rely on the digitizing result, it might fool you.

One thing of interest, replace the photo diode with a 70pF cap and see whether you pick up the same noise.

18. Feb 25, 2012

### Artlav

50Hz, mains hum.
At 100 samples/sec it was synced up almost perfectly, giving ~2Hz from drift, but at higher and non-divisible read-out frequencies it's plain and obvious.

It's RF - any light source indoors - hum.
Outdoors - no hum, no detectable noise at any light level with a flashlight.

How do you filter out mains hum...?
Any better ideas than a Faraday cage?

19. Feb 25, 2012

### yungman

Any way to take pictures of the real circuit and the breadboard? One look might worth more than a thousand words. 50Hz, are you in Europe? I can't even get a sandwich out of you even if I can help....you too far away!!!!:rofl:

20. Feb 25, 2012

### yungman

Hold your horses!!! Did you do what I asked to tape the window of the detector diode? If you did not, the 50Hz is likely from the light!!!! Forget the picture, answer this first.

If it is from the light source to the detector, you need to talk to the one designing the optics. If that cannot be avoid, you might have to do cancellation. I did it on one of the transimpedance amp that has switching supply noise. We needed a Faraday Cup amp floating on 200V. I decided to use a small switching supply build inside the FC amp box!!!!. To cancel the noise, take the ground noise and sum with the output of the FC amp and cancel or reduced the switching noise, it worked.

If so, and there is no way out as your signal has the 50Hz, then you might have to generate the 50Hz of opposite phase and sum to cancel the noise.

21. Feb 25, 2012

### Artlav

Both, actually.
With any mains lights (bulbs and lumenescent ones) i always get noise to one degree or another.
In total darkness there is no noise unless i get right next to a fridge or something.

With a flashlight i get various amount of noise depending on where i stand - away from any wires there is no noise.
It's also light level dependent - if there is no light the noise is tiny, but if i shine a flashlight while in the same spot, it can be +-0.5V at the output.

Weird, i thought incandescent light bulbs didn't pulsate.
Not sure yet if it's a problem or a result.

Everything is battery powered, so as far as i understand there is no reference to cancel it out on the analog side.

More or less - i'm in the western part of Russia.

22. Feb 25, 2012

### Antiphon

First things first- your diode in the schematic looks reversed. If you are using a one-sided supply this will give erratic results.

23. Feb 26, 2012

### dlgoff

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24. Feb 26, 2012

### yungman

1)Well so much for the idea of power supply. So the unit is totally isolated from everything.....it's a handheld? But how do you measure the 50Hz? You must have to hook up the scope ground!! Tell me more about your device if you need help as I can't trouble shoot with the info given. How do you digitize and get the result? Is all the digitizing circuit in the same unit and all power by battery?
2) Want to verify that if you tape the diode, you don't get any noise unless you are close to the fridge.
3) If you are in the room that you have lights on, you get 50Hz, but if you leave the unit in the exact place and turn off all lights, the noise goes away?
4)Then with the unit at the same place as 3) with no lights on, but when you shine a flash light, the noise come back even the room light is off and is total darkness other than the flash light?

1, 2 and 3 points to the 50Hz is from the light source. BUT 4 don't make sense at all. Assuming the flash light is battery powered, you should not have 50Hz.

Take a picture and post it anyway.

I don't think the noise is electrical, but humor me, use some kitchen aluminum foil and cover the circuit and make sure it is grounded. See whether there is any improvement. At least whether you get rid of the noise when you are close to the fridge with the diode taped.

What is your application, do you have to work in the lights? If all else fail and the 50Hz is from the light source that you have to work with, then:

1) can you lower the bandwidth to below 25Hz? If so, at least you can use steep low pass or notch filter the get rid of the 50Hz.
2) If you need to cover to over 50Hz, active cancellation is the way to go. Set up another amp with diode but point the diode at place that you are not measuring but still can detect light and sum.

Last edited: Feb 26, 2012
25. Feb 26, 2012

### Artlav

I've mentioned it somewhere in the thread- it should have been drawn other way around.
I'm using it in photovoltaic mode, not backwards.

In the output.
Most of it seems to come from the lights.
None if the PD is covered, unless really close to the wires or motors of some sort.
None at all in the sun outside, little to none in sunlight inside, as above dependant on the presence of wiring.
So far that's fine for me.

Thanks for help so far, i'll post in details if i run into something unsolvable.