Author Topic: Help with opamp for shaping circuit  (Read 7511 times)

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Offline oschonrock

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Re: Help with opamp for shaping circuit
« Reply #25 on: August 10, 2020, 08:12:54 pm »
Lets say the avalanche is instantaneous for a moment ... the original waveform is already the output of a low pass filter for that instantaneous energy.

In principle it's all linear ... the peak of the low passed pulse has a linear relationship with the peak. Also in theory the area under the curve is linear with the original energy too, which can be measured with less noise than the peak (and with a lower resolution ADC). Not that noise seems to be a problem here.

That's "possibly correct", but I am not sufficiently familiar with the physics apparatus he is using. In his response above he says "Yes, correct the shape of the pulse is irrelevant, only the peak value is of interest", and he is the best one to judge given we don't know he experimental setup...?
 

Offline oschonrock

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Re: Help with opamp for shaping circuit
« Reply #26 on: August 10, 2020, 08:14:45 pm »
Meh, don't make things harder than they need to be ... just R-C filter it, then a PGA.

I agree "shaping it into a gaussian" does not seem relevant, but upon request for clarification he clearly says: "Yes, correct the shape of the pulse is irrelevant, only the peak value is of interest"...

Which means.... if you RC filter it, you have just ruined the measurement..?
 

Offline oschonrock

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Re: Help with opamp for shaping circuit
« Reply #27 on: August 10, 2020, 08:34:17 pm »

I've captured the two pulses for you. One rather small one of 140mV, the other 850mV. In the final version, the signal will be less noisier than in the scope shots, i basically used the voltage from a bench-top supply and put things on a breadboard with a not-professional coupled detector, so ignore the noises

That's very helpful thank you.

Without doing a fourier analysis on that, my MKI eyeball says that the leading edge of those pulses "fits" a sine wave of half period of about 3us. ie 6us full Period or about 160kHz. That means that high speed peak detector circuit should be able to keep up with that pulse (or very nearly so).

I am not sure what your constraints are, in terms of budget and time. I would do the following:

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?
« Last Edit: August 10, 2020, 08:38:48 pm by oschonrock »
 

Offline OM222O

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Re: Help with opamp for shaping circuit
« Reply #28 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.

If you're interested in pure sampling, then you need some form of formula that describes the overall shape of the pulse (maybe split it into 2 for conviniance, it seems like a sharp linear rise, then a decay process). You can then use the "18 samples" to figure out the scaling / offsets that when applied to the basic formula, reconstructs the original curve, allowing you to analyze the data.

Solution to the variable gain is already answered and as everyone mentioned reshaping doesn't seem to do anything in this case. Please read through all the replies carefully, you should already have your answer
 

Online Kleinstein

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Re: Help with opamp for shaping circuit
« Reply #29 on: August 10, 2020, 09:12:01 pm »
Meh, don't make things harder than they need to be ... just R-C filter it, then a PGA.

I agree "shaping it into a gaussian" does not seem relevant, but upon request for clarification he clearly says: "Yes, correct the shape of the pulse is irrelevant, only the peak value is of interest"...

Which means.... if you RC filter it, you have just ruined the measurement..?
One does not need to directly read the hight 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).


If the pulse shaping / filter will remove the frequencies beyond the Nyquist limit, it should be OK to get the pulse area from a relative limited number of points. One has 2 options here:
One is integrate over a little more than the actual lenght, so that getting the exact position does not matter that much. However this would add some extra noise.

The other way, to get close to minimal noise would not calculate the area, but do a correlation with a test function that is similar to the peak shape (probably just a sequence of some 50 values). This way the values closer to the peak maximum get a higher weight and the points at the start and end get little weight. So there is little error from missing a few point at the start or end. Compared to just looking for the largest point, one still uses more points and is less sensitive to having a sample just at the peak.
 

Offline oschonrock

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Re: Help with opamp for shaping circuit
« Reply #30 on: August 10, 2020, 09:18:43 pm »

One does not need to directly read the hight of the peak.

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.
 

Offline oschonrock

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Re: Help with opamp for shaping circuit
« Reply #31 on: August 10, 2020, 09:21:46 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.


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? ;-)

 

Offline Marco

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Re: Help with opamp for shaping circuit
« Reply #32 on: August 11, 2020, 12:08:28 am »
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?

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.
« Last Edit: August 11, 2020, 12:10:31 am by Marco »
 

Offline snxTopic starter

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Re: Help with opamp for shaping circuit
« Reply #33 on: August 11, 2020, 05:17:41 am »
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).
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!

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?

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.
 

Offline OM222O

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Re: Help with opamp for shaping circuit
« Reply #34 on: August 11, 2020, 11:36:14 am »
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.
 

Offline snxTopic starter

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Re: Help with opamp for shaping circuit
« Reply #35 on: August 16, 2020, 07:43:01 am »
Quick Update:

Parts for the test will arrive next week and also a development board for the bias. I will then assemble the peak detector and will show results
 


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