LED on Photodiode step response

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I removed the PD and sent the (50ohm terminated) voltage pulse through a 100k ohm resistor into the inverting input. Beautiful square looking step at the output.

Whee... after turning off the room lights, the 'laying on of fingers' did nothing for the slow tail. It did slow down the fast response.

George H.

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I've used a few different configurations... all give the same result. Coax from the signal generator, daisy chains to the 'scope and on to the the LED. There I had 50 ohms to ground and in parallel 500 ohms feeding the LED to ground. Originally there was a few inches of wire between the 500 ohm resistor and the LED, but I later tightened it up and put everything right at the LED. I later just feed the LED through a 50 ohm resistor.

The PD circuit is standard TIA

I'm going to write to OSI (the PD maker.) I wanted to be sure I wasn't doing something stupid beofre contacting them...

George H.

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Not mine, just from google.

It probably would, and if your driver can sink as well as source current, it's just a matter of putting a resistor-capacitor series pair in parallel with the current limit resistor.

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Yeah perhaps more like 3.3 pF across the feedback.

The pulse response through a 'big' resistor looks just fine, (no PD or LED). I went back and added the 'missing' 12 pF to ground, just in case...(That's the removed PD capacitance, the strays and opamp C are still there.)

Hey this might be some 'real' physics and not just a circuit screw- up.

George H.

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Oh, I did that Friday. The PD is reversed biased 12 V, then through

10k ohm with a buffer looking at the top of the 10k.

Here's the fast response... slower than without the TIA.

And here's the slow time.... The long tail remains!

George H.

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Thanks John, I've got lotsa laser diodes. VCSELs sound expensive. Who sells them?

George H.

Do you have any kind of a chopper or shutter that you could use to isolate the time constant(s) of the PD frome those of the LED/ laser?

This thread really has been off the charts in the unwillingness of posters to trim out unnecessary cruft when they reply!

Nah, that's situation normal on a sci.electronics.design. Even the flame wars are like that. :(

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

Well, it does look like the photodiode, which suggests that the aforementioned diffusion tail mechanism is doing it. I'm surprised that jacking up the bias didn't fix it. It isn't usually as bad as that.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

[...]

They're quite affordable, comparable to laser diodes really depending on type of course.

I bought some from Roithner (European company, they have a huge range of laser diodes and leds). Looks like cheapest is $9.25 for PM85-D1POU (2mW 850nm with dome lens, 2 degree beam).

Just remembered there are also "resonant cavity" leds, similar pricing, these are also designed for fast modulation (3ns rise/fall times on the one I am looking at now). Don't know really what their noise is like but being "leds" I would guess they are better than laser diodes.

--

John Devereux

Nice. Any hassles with end-use certificates or do they just ship whatever you order asap?

Get some scroll bars. They're not expensive.

John

It would be interesting to see how fast an edge you could create with a mechanical chopper. Nanoseconds would be tough.

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Yeah, I was thinking about sweeping a laser across the PD face, with a rotating mirror or something. But then it's real optics and not just sticking an LED against a PD.

George H.

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Sorry AES, "Twas I", that crossed posted to both sci.optics and SED (sci.electronics.design).

Next time I want to ask both groups the same question, I'll start separate threads.

George H.

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Yeah, but I'm still confused. I tried it with an adjustable bais (from 0-12V) late today. There was the expected change in the short time response. But none in the long tail. I had a nice chat with someone at OSI. (sent pic's and schematics to them.). I'll have to try a different PD.

George H.