Re: photodiode dark current at zero bias
- From: Phil Hobbs <pcdh@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Wed, 28 Sep 2005 11:24:28 -0400
Adam S wrote:
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
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.
300 uV/200 pA = 1.5 Mohm, which is certainly on the low side even for a big PD. What exactly are you using for a PD, and does it have a spec for zero-bias resistance? (A solar-cell type diode might very well have a zero-bias resistance of this order.) A definitive test for PD leakage would be to build a voltage divider, using a 10k resistor to a variable *negative* supply voltage, and a 10-ohm resistor to ground; connect the PD to the tap and see if you can make the output move around by changing the PD bias by a few millivolts.
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.
3. (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.
4. (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 .
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