Re: Photodiode Question

On Tue, 23 Mar 2010 12:59:12 -0700 (PDT), George Herold
> > > > > > wrote: > >> >> 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: > >> >> >> > > In article , > >> >> >> > > =A0Phil Hobbs write=

> >> >> >> > > > =A0Night lights use CdS or CdSSe photoconductors, but they=

don'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 illuminatio=

n history.)

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

tory?

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

ate spam.

> >> >> >> > Hello, > > >> >> >> > Go towww.perkinelmer.comandsearchfor"photocell", or follow the > >> >> >> > links. When at the photocell page, on the right side you have =

>> >> >> > application note link. They have a very nice document on the L=

DRs 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:
formatting link

alogOptoiso.pdf

>> >> >> you can find the document about LDR. I will leave it there for a=

bout

>> >> >> 10 days. > > >> >> >> Best regards, > > >> >> >> Wim > >> >> >> PA3DJSwww.tetech.nl-Hidequotedtext - > > >> >> >> - Show quoted text - > > >> >> >Thanks for the document, page 35 talks about shot noise in LDR's. =

=A0Are

>> >> >they just throwing equations around or is there shot noise in LDR'=

> >> >> >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 nois=

>> >> 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 seri=

>> >> 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 curre=

>> >> with low noise. Low voltage across a low-value metal-film resistor > >> >> doesn't work because of Johnson noise. High value resistors are noi=

>> >> in their own right. > > >> >> John- Hide quoted text - > > >> >> - Show quoted text - > > >> >"> Since photons make current, probably so. But LDRs are so messy it'=

>> >> 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. =A0I 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. > > 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 =3D > ADA4817. I figure we'd measure from 1 KHz to 100K maybe. > > John- Hide quoted text - > > - Show quoted text -

Hi John, The current source is just a 10 Volt reference (REF102) feeding the big resistor with an opamp on the bottom driving the 'ground' pin of the reference. The references has a bunch of noise so I'm not sure how quite the current source will be. We'll find out though.

The ADA4817 is that the screaming 1GHz FET? I was going to use the much slower opa134.

George H.