photodiode shunt resistance equation

Look for the (non-photo)diode equation.

Thanks for the reply. It's probably that simple and staring me in the face. I know the diode equation I don't see how to relate it to photo diode shunt resistance so I can get the shunt resistance noise. If I use ohms law and the bias voltage is zero I'm back to square one.

At one point Aldert Van der Ziel in his book converts the dark current noise into a conductance to simply his model.

Idark * q/2kT

Is that 1/Rshunt? Is the shunt resistance just a different way to write the dark current noise?

Thanks

Wanderer wrote: (snip)

I don't think so. The incremental shunt resistance is the slope of current as voltage changes. This is in addition to the approximately constant (but noisy) current that is almost independent of voltage but very dependent on temperature.

Dark current noise in photodiodes is almost never the limiting noise source in a measurement. What are you actually trying to measure?

Cheers,

Phil Hobbs

I'm building an incoming inspection tool for one of these

I'm computing the noise through a power spectral density matlab program. I see the dark current drop like it is suppose to, but I only get 6% change in noise from 25C to 5C. I'm told I should see a much greater change because of cooling the shunt resistance. I know it's just some bugs in my code, but I'd like to do some calculations to get an idea of expected results.

Thanks

I don't know. Karl Spangenberg has an Appendix on shot noise in Fundementals of Electron Devices 1957(I have a bad habit of collecting old electronic books, though sometimes their better than the internet). He takes the derivative of the diode equation and gets a conductance qI/kT and calculates the rms noise to get 2qIB. Though he adds the disclaimer:

"The above demonstration is not really a derivation but rather a rationalization. It assumes that the phenomenon giving rise to resistance noise is the same as that giving rise to emission noise. It will at least make the formula seem reasonable and dimensionally correct."

"Wanderer" schrieb im Newsbeitrag news: snipped-for-privacy@b29g2000hsa.googlegroups.com...

Hello,

How do you calculate the noise?

Is it the integrated noise voltage over a certain bandwidth?

What's the lower and upper frequency limit of your measurement and noise calculation?

I remember that Rshunt will fall by a factor of two for every temperature rise of 6 to 10 degree.

Best regards, Helmut

Judging from the very high feedback resistor and the rapidly rising noise curve on Page 2 of the datasheet, that gizmo is dominated by op amp noise above about 10 Hz.

The photodiode capacitance and the feedback resistor form an RC differentiator that multiplies the voltage noise of the op amp, and (with this device) once you get as far as the audio region, the PD noise hardly enters the picture.

Cheers,

Phil Hobbs

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I take the FFT over 100hz bandwidth convert it to a Power Spectral Density by taking the absolute value, throwing away half , multiplying by 2, etc. I then have a function which breaks up the data into different bandwidth regions, sums the points in the different regions, divides by the number of points in the region, by the bandwidth of the region,divides by 2 and takes the square root. I've been experimenting with different size regions.

It's a low frequency dc application. I'll try focusing my analysis below ten Hz. I've been focusing around 50Hz because I don't trust the edges of the window.

Thanks