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Re: Photodiode Question

On Tue, 23 Mar 2010 10:39:15 -0700 (PDT), George Herold

> > > > > > wrote: > >> >> On Tue, 23 Mar 2010 06:59:08 -0700 (PDT), George Herold > > >> wrote: > >> >> > >> >> > On 22 mar, 23:52, snipped-for-privacy@ip-64-139-1-69.sjc.megapath.net (Hal > > >> >> > Murray) wrote: > >> >> > > > > >> >> > > > =A0Night lights use CdS or CdSSe photoconductors, but they do=

n't have to

>> >> > > >have any kind of stability or repeatability, and they don't. =

=A0(CdSSe's

>> >> > > >resistance can vary 5X due entirely to previous illumination h=

istory.)

> >> >> > > What is the mechanism for that history quirk? > > >> >> > > What is the time scale? =A0How long does it remember the histor=

y?

> >> >> > > -- > >> >> > > These are my opinions, not necessarily my employer's. =A0I hate=

spam.

> >> >> > Hello, > > >> >> > Go towww.perkinelmer.comandsearchfor"photocell", or follow the > >> >> > links. When at the photocell page, on the right side you have an > >> >> > application note link. They have a very nice document on the LDRs=

that

>> >> > includes the memory effect also. > > >> >> > Best regards, > > >> >> > Wim > >> >> > PA3DJSwww.tetech.nl > >> >> > without abc, PM will reach me > > >> >> The document that I have is no longer present on the Perkinelmer > >> >> website. Here:
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gOptoiso.pdf

>> >> you can find the document about LDR. I will leave it there for abou=
t
>> >> 10 days. > > >> >> Best regards, > > >> >> Wim > >> >> PA3DJSwww.tetech.nl-Hidequoted text - > > >> >> - Show quoted text - > > >> >Thanks for the document, page 35 talks about shot noise in LDR's. =A0=

Are

>> >they just throwing equations around or is there shot noise in LDR's? > > >> >George H. > > >> Since photons make current, probably so. But LDRs are so messy it's > >> likely buried in other gunk. > > >> One of my guys is building a test setup to measure shot/excess noise > >> in resistors. We need a ~~ 100M resistor to create a small (50 nA > >> maybe) bias current with below shot-level noise. It's not clear if > >> high-value cermet resistors have shot noise or not, so we plan to > >> measure a bunch. > > >> Metal film resistors don't go to very high values; we can probably get > >> 10M and likely 22M, and maybe even 50M, so we may have to do a series > >> string. Even then I want to measure them to make sure they behave. I'm > >> guessing that axials are better than surface mount, because of the > >> bigger available surface for depositing metal. > > >> It's an interesting problem, trying to generate a nA-range DC current > >> with low noise. Low voltage across a low-value metal-film resistor > >> doesn't work because of Johnson noise. High value resistors are noisy > >> in their own right. > > >> John- Hide quoted text - > > >> - Show quoted text - > > >"> Since photons make current, probably so. But LDRs are so messy it's > >> likely buried in other gunk." > > >Hmm, Yeah this is a bit confusing. =A0The photons make the charge > >carriers and reduce the resistance... and as Phil H. says there is > >generation and recombination noise. =A0But this should only depend on > >the light level and not on how the device is biased. =A0For a given > >light level if I double the bias voltage (and thus double the current) > >does the noise go up? > > What's amazing to me is that there are resistors that *don't* have > shot noise. > > Any time you have a current in which the electrons arrive randomly, > there's shot noise. It's pure raindrops-on-the-roof statistics. That > happens when the electrons are knocked loose by photons or when some > thin barrier, like a P-N junction, doles out electrons across a > surface. Metal wire is unique in having electron interactions that > smooth out the flow. As far as I know, all semiconductor junction > currents and leakages have shot noise. Tubes have shot noise. > > The shot noise current depends only on the average current; it goes up > as the square root of I. Of course, some devices have more noise than > pure shot noise. > > > > >I'm using 100M and 1 G resistors from Ohmite to make a 10nA current > >source. =A0I'll look at the noise next chance I get. > > I'd appreciate that. We'll share whatever we learn. This is not very > easy to measure. > > John- Hide quoted text - > > - Show quoted text -

"> I'd appreciate that. We'll share whatever we learn. This is not very

easy to measure."

Hmm I hadn't thought it would be that hard... but then I'm probably missing something. I was going to take the the 10nA current source and fed it into a TIA opamp circuit with 100Meg as feed back resistor, (giving me a volt of DC across the resistor) And then see how noisy it is. I can then compare it to 10 nA's from a photodiode. Do you care about high frequencies (above 10kHz-100kHz or so..) or very low frequencies? (There's 1/f noise in the FET opamp that starts to interfere at the low end.)

Our noise apparatus is on a truck somewhere between here and Portland, it may be back tomorrow though.

George H.

Reply to
George Herold
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Fine, as long as that 100M resistor has no excess noise of its own, and you account for its Johnson noise, and the TIA has very little input current noise. And all the supplies are very quiet. And everything is really well shielded.

We're going to make a voltage divider from two identical RUTs, and AC couple that into an opamp with a gain of +1000. The AC coupling RC will add shunt Johnson noise but no excess noise of its own. We're figuring on a polystyrene cap and a 1G resistor maybe. Opamp = ADA4817. I figure we'd measure from 1 KHz to 100K maybe.

John

Reply to
John Larkin

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If you use two FET op amps going into two scope channels, and use the math functions to multiply the two inputs, FFT the results, and average over N traces, you'll get just the noise of the resistors, because everything else cancels out. How big N is depends on how far below the amplifier noise you want to go.

Quiet power supplies are of course vital.

Sounds like a really interesting measurement.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
Reply to
Phil Hobbs

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Sounds like a lot of math. That would be good for extremes, like cryogenics maybe.

Why FFT? Wouldn't the averaged product work?

The ADA4817 has 4 nv/rthz noise, and the Johnson noise from 50M will be almost a microvolt per, so we should see the noise pretty well. Input noise current is low enough to not make trouble. It's an amazing opamp.

If cranking up the DC voltage into the divider doesn't much increase the noise, we're done. If we jam 20 volts into the 100M:100M divider, full shot noise would make ... calculates furiously ... 13 uV/rthz, a huge signal.

John

Reply to
John Larkin

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Yeah I'll have to check that the 100M is OK..you can always switch down to 10 and 1Meg.... You can measure changes in noise down below the amplfier noise...at 10% of amp noise it's still pretty consistant. I think the 100M will dominate the input amp noise.

I told my boss that the capacitor multiplier was my latest hammer. I've been hitting every noise nail that I find with it. (Within Teachspin this circuit fragment is called a Hobbs filter, because I first learned of it from Phil's book.)

I've got a five sided steel box with an aluminum front panel. I sorta gave up on magnetic shielding and we just try and keep all the magnetic crap away from the front end. The power supply is a brick on a rope that can live far away. Then keep the 'scope on the far side of the lab bench.

George H.

Reply to
George Herold

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Cool, the second opamp is looking at the voltage source feeding the ladder?

George H.

Reply to
George Herold

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Yeah I think things will be fine. (Though I really have no idea.) Your resistors may have 1/f noise but there shouldn't be any shot noise.

Still, I must order some CdS LDR's. I still don't get the shot noise.... and to paraphrase Brian Pippard in the intro to "The Physics of Vibration" "I'm not as smart as some others and it's nice to have some experimental results to help guide my thinking."

George H.

Reply to
George Herold

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If you connect a pin photodiode or a PMT, with a little gain maybe, to a scope, and hit it with a light pulse, the top of the pulse will usually be a lot noisier then the baseline (dark) level. It's pretty obvious if the light is in the roughly 1 microwatt sort of range. It's the quantized photons hitting the detector randomly... shot noise.

I guess it was named for the sound of shot (as in birdshot) falling onto a surface. Hisssssss.

John

Reply to
John Larkin

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