EL7900

Il 28/08/2012 20:52, Phil Hobbs ha scritto:

I found these ones:

If I understand correctly:

- the reverse light current is independent of the reverse voltage until the diode is properly polarized. What is the minimum recommended reverse voltage?

- the maximum light I can measure is only limited by the thermal dissipation (Ilight * Vr).

Regards Marco

cents.

eets/1244281.pdf

/a lurker/

If you are going to use a TIA opamp, then reverse biasing only helps get you a bit more speed (by reducing the PD capacitance) With the stray C to all the other PD's this is not going to be of much benefit. So (I think) Phil H. was suggesting to just leave the non- iverting input grounded and switch PD's in with an open drain or tri- state output on the shift register (SR). (His original circuit with the SR just changing the bias condition of the PD's was a (very rare) mistake. He caught it himself, before we dog- piled on him.)

George H.

This is just another goofy idea, but you could connect a capacitor across each photodiode. A scanning analog multiplexer would hit each pd/cap array in sequence, simultaneously measuring how much integrated current the PD had detected, and recharging the cap. I guess that CMOS and CCD camera imagers work that way. One 8:1 analog mux per 8 PDs isn't a lot of overhead.

Reminds me of the difference between Farnsworth's image dissector tube and Zworykin's iconoscope; the image dissector essentially multiplexed the photocurrents of a pixel array, and the iconoscope multiplexed the integrals of the pixel currents. The s/n advantage of integration was huge.

ew cents.

se

Well and one cap for each PD. And as long as you don't wait too long to read out, and let the cap charge up so that it starts to forward bias the diode. I guess if you found a cap that was 'fully' charged you could increase the readout rate. A software controlled 'gain', which is nice.

Yeah, seems a shame to only use the photons that are hitting the PD while you are looking at it.

George H.

.highlandtechnology.com=A0 jlarkin at highlandtechnology dot com

Il 29/08/2012 18:13, George Herold ha scritto:

On my board I have plenty of photons, no problem to lose some ;)

Marco

The lensed ones don't have uniform angular response, so you might want to look at this sort of thing:

A lot depends on your linearity spec. A bit of reverse bias helps, but you probably don't need more than a couple of volts.

Cheers

Phil Hobbs

ohm.

If you look at that inordinately low pA dark current spec, low for a MOSFET= anyway, it means they're doing something unusual. My guess is this is achi= eved by keeping the output FET operating sub-threshold all the time (achiev= ed by doping for a high Vthreshold), and in this region the IDS vs VGS oper= ation is exponential, not at all like the resistive-triode characteristic. = So an abrupt turn-off in the vicinity of compliance voltage vs Iout would n= ot be all that strange, just a few 10's millivolts will do it.

I'm actually in the middle of doing a photon budget shootout for a customer with some rather special requirements, between integrating and resistive front ends. All the noise sources are the same, except for the kTC noise on the one hand and resistor Johnson noise on the other, but in practical circuits the tradeoff is fairly non-obvious. Of course that's with one amp per PD, which isn't the same as the TV camera or the OP's situation.

The OP probably wants to avoid adding another 400 parts to his board, which is a big hit even by my standards. ;)

Cheers

Phil Hobbs

Quad c-packs?

Looking at only the first page, the answer / problem is that the amplifier saturates; no need to go further..

As I pointed out early on. But lurkers here would rather kiss-up to Larkin than deal with reality. Fortunately, kissing-up does not pay very well ;-) ...Jim Thompson

It's not behaving like a saturating amplifier; it's folding back down to zero output as the light input increases. And, apparently, much sooner than would be expected from the data sheet.

The pfet current mirror model explains the effect, but not why it does it at low output currents.

Il 29/08/2012 20:25, Phil Hobbs ha scritto:

Yep, you're right :)

Marco

Il 30/08/2012 05:06, John Larkin ha scritto:

Exactly.

Marco

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It's not the output current - as such - that's the problem, but the light level.

The P-channel MOSFET current mirror model explains the effect, but without data on the specific P-channel MOSFET involved it can't say anything about the current level where the tail MOSFET switches from being a resistor to being a constant current source.

All we know is that the P-channel MOSFET's in the production batch from which Marco Trapanese bought his parts switched at a rather lower current than the batch the manufacturer tested to generate the data- sheet.

Whoever wrote the data sheet has got egg all over their face, but that's human error, and it happens from time to time.

-- Bill Sloman, Nijmegen

You would think that the parts would be tested in production.

(1) It's ALL P-channel, mirror AND enable devices (2) The mirror is 1:M (3) Crank it up to Vcc=+5V and see what happens (4) Refer to:

Date: Tue, 28 Aug 2012 11:36:15 -0700 Message-ID: ...Jim Thompson

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Obviously not at high light intensities ...

-- Bill Sloman, Nijmegen