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Photodiodes saturating in ambient light
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twospoons:

--- Quote from: NiHaoMike on May 14, 2020, 01:37:47 pm ---
That configuration will magnify the offset voltage of the opamp. Easiest fix for that is to connect the photodiode directly. Then add an integrator and another resistor to cancel out DC.

--- End quote ---

Yes it will, but its fairly easy to pick a low offset opamp and/or add offset trimming. At least it will be fairly constant, unlike the ambient induced DC offset.  Or simply live with it, by splitting the amplification into several AC coupled stages.

Now we're heading into the 'art' side of engineering: one problem with multiple solutions.
aussie_laser_dude:
oh snap....  :'(

Yeah I didn't give much thought to the dc top of the square wave pulses being filtered out, (with hindsight) that's obviously going to be removed with a high pass filter. oops lol.

Art hat on:
Option A)
Take two signals,
1. raw photodiode signal
2. a slow changing filtered signal made via a cap / inductor to measure ambient light
Then subtract them and amplify using an op amp to get square wave pulses with removed ambient DC current.

Option B)
Feed the highpass filtered signal into an amplifying & integrating op amp, that'll reconstruct the square waves with no dc offset i think? This approach sounds fun (oh, NiHaoMike already had a similar, probably better idea that I missed)
 
twospoons:

--- Quote from: aussie_laser_dude on May 15, 2020, 03:21:46 am ---Yeah I didn't give much thought to the dc top of the square wave pulses being filtered out, (with hindsight) that's obviously going to be removed with a high pass filter. oops lol.

--- End quote ---

wait .. what?

What squarewave pulses?

If you want to pass a squarewave, and block DC, you want a highpass filter - with a cutoff below the fundamental of the squarewave. Its all the higher harmonics that make it square and flat across the top.
nfmax:

--- Quote from: Siwastaja on May 13, 2020, 10:59:08 am ---IMHO, don't waste time on polarizing filter, you may be able to get some (like maybe 2-3x?) of extra signal-to-ambient ratio out of it, but with a simple bandpass filter you easily get 20-30x!

Yes, laser is heavily polarized and ambient light mostly isn't, but laser is also a small peak in wavelength, which ambient is not. The latter feature is easier to extract a lot of benefit out of it. If you want to use polarization to perform attenuation of ambient, the physical alignment of source and measurement becomes critical.

--- End quote ---

I suspect the OP is well aware, but others may not be, of the fact that a narrow-band dielectric optical filter will only give the desired response for light incident normally on its surface. The sunlight may come from any direction, which may give problems. But it sounds like you have so many wanted photons, even the shot noise from the sunlight isn't a big issue. Luxury!


P.S. Get a copy of Phill Hobbs' book. A third edition is in preparation and may be published this year, but I wouldn't wait
tggzzz:
I realised nobody has posted a link to Phil Hobb's site on this thread. 18 hours ago I posted the info below on another thread on the same topic...

You should invest in Phil Hobbs' book https://www.electrooptical.net/Building_ElectroOptical_Systems/

Alternatively, if you can ask a good and interesting question which doesn't look like you are merely after free consultancy, he may respond on usenet group sci.electronics.design.
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