photodiode dark current at zero bias

Silicon photodiodes are not sensitive above about 1100 nm. The peak (in amps/watt) is typically around 950 nm, which could be shifted if filters are used but it can't go above 1100 nm (photon energy becomes less than band gap energy).

This is all way below room-temperature thermal IR.

Mark

I've also had problems connecting a photodiode directly to an op-amp. Mind you, these were just cheap 741's. But I did find that putting a small resistor between the photodiode and opamp helped (I think I used

Try a resistor in series with the photodiode.

Mark

and most have to be used with a blue filter which reduces transmission ...

have you also checked to see whether the opamp itself is oscillating ? that is, this in itself can cause offset problems -- the photodiode's junction capacitance is related to the surface area so the opamps get a little troublesome to compensate. check out Burr Brown (TI's) website for ideas on compensating photodiode amplifiers.

I believe the sensitivity of a silicon junction has a peak in the (high) IR region...

And if you could put a pv device inside a box, and have it generate voltage from the IR emitted thermally by the walls of said box, it would violate conservation of energy and a law or two of thermodynamics.

John

Hmm. No, that doesn't seem right at first blush. How about the following?

1. (Tire-kicking) I assume you're using the usual op amp TIA, with the + input and the PD anode grounded, the - input connected to the PD cathode, and a big feedback resistor connected between the - input and output of the op amp. How big is the feedback resistor, and how are you measuring the output voltage? Scope? DVM? ADC card and Labview? (involuntary shudder) Are you using split supplies, or an op amp with common-mode range including the negative rail? How sure are you that the input offset voltage is < 300 uV at your actual bias point? (You probably didn't learn that by hanging a DVM off the summing junction.)
2. (PD details) If you're trying to detect very low levels, you're probably using a huge PD, right? Diodes at zero bias have a large but finite resistance.

Does warming the PD very slightly make it noticeably worse? Besides the zero-bias resistance, the thermocouple potential between silicon and any metal is gigantic--400 uV/K or thereabouts, which under some circumstances might dwarf the 300 uV input offset voltage.

1. (Light leaks) Or you might be using a plastic cover that looks black but lets 800-1100 nm light pass through--the window filter of an IR remote control is like that, and it's not unknown in other plastics--some very early plastic IC packages had that problem, in fact. If your box is plastic, try wrapping it in aluminum foil. In fact, try wrapping it in aluminum foil anyway, to make sure it isn't light leaks that you're seeing.
2. (Instability) Make sure you have a capacitor in parallel with your feedback resistor, to reduce the noise BW and eliminate the instability that the other guys are warning about--though I'd expect an oscillation to result in at least hundreds of millivolts of dc shift, even if you're measuring the output with a DVM.

Cheers,

Phil Hobbs

I'm trying to detect very low levels of light using a PN photodiode. The photodiode is connected to input of a CMOS op-amp and the bias voltage is less than +-0.3mV. When I remove all light by placing the detector in a sealed box, there still remains about 200pA of output current.

Disconnecting the photodiode showed the circuit stray leakage current to be less than 5pA, so that cannot be blamed.

My question is does dark current supposed to exist at zero bias ? If not then is the current caused by IR radiation from small temperature differences between the photodiode junction and surrounding surfaces ?

Adam

Nope!

Or do you refer to Johnson noise? Or have you invented perpetual motion?

John

Thanks Phil, for your 4 points. I just logged on to reply with the problem I found causing the small DC shift of ~20mV. Instability.

After attaching a CRO to the output of the op-amp I noticed an amazingly high frequency 20mVpk-pk signal, almost in the VHF range. Who would think a slow GBW 1.5MHz LMC6482 could cause such a thing ? Actually I think the oscillation was due to the 2nd op-amp in that package which was driving a light feedback stabilized LED circuit. Adding some bypass capacitors fixed the problem.

The output voltage was measured with DVM, the op-amp feedback was 20Mohm in parallel with 47nF.

Actually, perpetual motion was invented the day that politicians were invented...

Politicians weren't invented. They are the spawn of satan.

Well, it's obviously Doomsday. Here, in one day, I've found myself: (A) laughing out loud at a joke that made Jim Thompson Sno-o-o-ort, and (B) Agreeing with Michael A. Terrell about politicians.

Will someone please send me some better drugs?

Thanks, Rich

So who was the mom?

Cheers,

Phil Hobbs

Rosemary.

The Mother of Everything, of course: