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Brainstorm please! High speed (>20krpm) IR reflective optical trigger
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hans:

--- Quote from: jbb on October 20, 2018, 10:41:12 pm ---I guess that you’d have to measure the photodiode response with a pulsed circuit during design verification. Multi MHz bandwidths are possible using a fairly standard photodiode and TIA.

I think you could form a slit with a laser printer and an OverHead Projector (OHP) sheet (perhaps glued to something stronger) (also make sure the OHP sheet is rate to go through a prenter; you don’t want it melting!!).

If you want to get the highest bandwidth, here are some thoughts:
- photodiodes have stray capacitance which limits bandwidth
- if you bias them with a moderately high reverse voltage (could be 10 - 30V), the capacitance drops and bandwidth increases
- make sure the bias voltage is clean and well filtered because voltage noise will here will pass through the photodiode
- placing the TIA right next to the photodiode minimises cable capacitance and the area most sensitive to electrical noise
- don’t use excessive gain on the TIA because it will reduce bandwidth. You can add a second gain stage if you need it
- consider using an AC coupled gain stage to reject changes in DC levels. Then use a comparator to for the edges.

I think Analog Devices have app notes about this kind of thing.

--- End quote ---

And add a spot for a feedback capacitor parallel to the feedback resistor of the TIA.

The TIA will most likely cause severe peaking in bode plot (thus horrible ringing if you drive a square/pulse test signal), since the input impedance of a TIA looks inductive and the photodiode (and input of your opamp) is capacitive. The feedback cap adds a zero to the amplifier which you can tune to be just in front of the resonance frequency you may observe. That way you can get a flatband response.

You may still want to try several feedback resistors even if you're able to get -3dB at your desirable frequency. Sometimes you can get more gain (in absolute terms) but with a little bit more roll off (relatively speaking) at the highest frequency you need.

Be careful with the layout, because typically at several kohm's feedback resistance and several MHz means you only need a few pF of feedback capacitance to make it stable again.
max_torque:
All good stuff, thanks all   :-+

My current photdiode system is reverse biased at 20V (max allowed) and uses a TIA with an LT1226 amplifier.  The issue is that it's hacked together on stripboard, and i think that is the limiting factor (stray C's and L's)!!

Because the current system is reflective, and has about 3m of Plastic Optical fibre between the trigger disc and the diodes it needs to have a fair amount of gain, which is slowing thing down, and with the LED drive on the same bit of strip board, those high currents from the pulsing are causing havoc over on the sensitive photodiode amplifier) side..

With a continuous light source (led on all the time) there should be a lot less noise around, and with a interrupter type arrangement, i can get the LED and PD adjacent to each other (perhaps 3mm apart maybe), so get a much larger optical signal, meaning less gain required.

As mentioned, the waveform coming back from the PD is going to have a variable dc offset, and have short pulses with relatively fast edges when the trigger wheel teeth pass the sensor.  Ideally i'd want to have a circuit that works out the "no tooth" and "tooth" voltage levels, and perhaps sets a comparitor to one shot trigger at say 50% of that voltage, to try to get a repeatable, aligned edge?  I'd need to look into analogue peak hold circuits perhaps?  Depending on how the disc is arranged (ie is a tooth a "hole" or is a tooth a "not a hole") and depending on the average duty cycle of the disc (50:50 mark space ratio) perhaps i can recover an "average" voltage by just using a low pass filter, and then setting some hysterisis on either side of that value for the comparitor??
voltsandjolts:

--- Quote from: max_torque on October 21, 2018, 01:14:43 pm --- Ideally i'd want to have a circuit that works out the "no tooth" and "tooth" voltage levels, and perhaps sets a comparitor to one shot trigger at say 50% of that voltage, to try to get a repeatable, aligned edge?

--- End quote ---

Sounds like a data slicer.

max_torque:
ok, looks like the first job is to knock up a decent pulsed IR light source that can be used to determine the response of a detector circuit.  Be nice to get 50ns to 500ns pulses, adjustable, at perhaps a 1kHz rate, with the sharpest photon increase rate ?

Then, once i have that, i can develop a "constant" high intensity IR source, and a low latency / low jitter detector for use on the actual sensor. 

If i measure current into the driving led, and correct for the capacitance of that part, does current = photons?  (datasheet gives a quantum yield of 93%) ie i know the current and the voltage, so i can calc how many electrons go into the devices capacitance and how many get converted into photons and so i can estimate the IR intensity vs time during the pulse??
StillTrying:

--- Quote from: max_torque on October 23, 2018, 12:29:07 pm ---ok, looks like the first job is to knock up a decent pulsed IR light source that can be used to determine the response of a detector circuit.  Be nice to get 50ns to 500ns pulses, adjustable, at perhaps a 1kHz rate, with the sharpest photon increase rate ?
--- End quote ---

I'm glad someone else is going to do some LED fast flashing experiments. :)

There's some info here.
https://www.eevblog.com/forum/beginners/floating-probe!-for-$2-50/msg1834925/#msg1834925
edit. I've given up tryng to fix the link. :horse:

For very narrow pulses I use something like this, 10R will probably be too low for an IR LED's 1.25V.
I don't think you don't need to worry about the LED's physical capacitance, it only adds up to a couple of ns difference on the edges.
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