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Help with opamp for shaping circuit

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oschonrock:

--- Quote from: Kleinstein on August 10, 2020, 09:12:01 pm ---
One does not need to directly read the hight of the peak.

--- End quote ---

The OP says: He needs to measure the peak and only the peak. And you decide otherwise...? On what basis? I don't get it.

oschonrock:

--- Quote from: OM222O on August 10, 2020, 08:36:58 pm ---Snx please clarify: what are you trying to measure? If it's the peak, you can breadboard a prototype of peak detect for under 10$ and see if it works for you.


--- End quote ---

He did already clarify, because I asked:

https://www.eevblog.com/forum/projects/help-with-opamp-for-shaping-circuit/msg3182710/#msg3182710

The peak and only the peak.

I am really not sure where all this other stuff came from... area under the curve...we could also measure the shape of the clouds, because we think that's a cooler question? ;-)

Marco:

--- Quote from: snx on August 10, 2020, 08:02:04 pm ---Still, does this work with so few samples? Lets say we have 2MSPS, this means a sample each 500ns, so a pulse of 10us would get 20 Samples. Lets say for bad luck, the first and the last sample misses the pulse, so there is only 18 samples? Would that be enough to either find the peak, or calculate the area of the curve?

--- End quote ---

There's little difference really, either way you'd be fitting a curve using multiple samples. It's easier to think of it in terms of peak detection though.

You can do a fast analog peak detector and auto-reset circuit too, but it's going to be a little more complex.

snx:

--- Quote from: Kleinstein on August 10, 2020, 09:12:01 pm ---One does not need to directly read the height of the peak. It is good enough if the height of the stretched pulse is proportional to the hight of the input peak. To make sure this works the amplifier part should be quite linear, otherwise a RC filter is linear and will thus ensure the proportionality.
The gain factor for the whole setup would need to be calibrated from time to time anyway (e.g. with a known source or line from the background).

--- End quote ---
That is also correct. Maybe i need to clarify a few things more.

These detectors start conducting when hitting by light, so if placed between a positive and negative voltage, they actually produce something like short circuit. By Adding Series resistor and Output Capacitor you get a negative peak at the capacitor, because its stored energy is shorted to ground. See attached PDF for the detector circuits. I use Circuit E with a 1k Resistor and a 100nF Capacitor in the "Sout" line.
There are also possibilities to use different readout with Figure 13 with Trans-impedance Amplifier.

Actually what we measure, is not that much of importance. If its the pure peak, the area under the curve (this can be helpful in high pulse rate where pulses overlap, a peak detector cannot catch this), or the length of the signal. What matters is that the measured pulses have relation to the initial discharge on the detector. The SiPMS are basically the exact same thing than "regular" Photomultiplier tubes. For more information, see pdf from sensl.

Also see this page: http://physicsopenlab.org/2016/02/16/silicon-photomultiplier-sipm/ what they actually did there, is hook up one of those SiPMS to a Computer Audio Card as ADC to sample the pulse with 44ksps, and they produce rather good measurements for that experiment!


--- Quote from: oschonrock on August 10, 2020, 08:34:17 pm ---1. Make a protopye of that peak detect circuit. Feed your pulse through a unity gain inverting opamp circuit first (use a high BW/quality opamp, maybe the same type). I am thinking of a soldered circuit in your preferred format, not necessarily a PCB. You also need a mechanism for resetting the circuit between pulses obviously.

2. Then have a look on the scope. Can it follow? We think yes... Lets prove it.

3. At the same time, make some estimates of the inaccuracies of your aparatus / sensors. ie can you really get rid of that noise such that this is a 12-14bit signal? Get hold of a higher resolution scope if you can, to judge how clean this signal is. This is not that fast for a scope, so this should be possible. There is no point going nuts with 16bit ADC infrastructure if all you're going to measure is noise.

If the answer to 2 is "yes", then you no longer have a speed issue, and you can design your ADC stage for the accuracy which is justified by the answer to 3. (whether you need preamps / separate power supply etc).

I don't understand some of the other comments about RC / low pass filtering / estimate energy based on area under curve, etc. If I have understood your answer correctly, then you don't want to do that. I am taking what you said at face value. You want to measure that peak, as accurately as you can (just how accurate is yet to be determined).

Does that help?

--- End quote ---

Okay, i will try to build a prototype that is soldered to prototype board with the Analog Inc Design of the current boosted circuit. I have a few opamps and transistors here so i think i should finish this by the end of the week.

OM222O:
From the scope pictures looks like the signal is more on the order of 5 or 600Khz so there's a chance the current boosted circuit needs even more current boosting, for which there are plenty of things you can do. Just share the results from your prototype if it wasn't fast enough.

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