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Help with opamp for shaping circuit

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oschonrock:

--- Quote from: Kleinstein on August 10, 2020, 03:15:08 pm ---
The peak shaping is nothing magic, it is essentially a low pass filter with a suitable response in the time domain, so that the short pulse will get wider, as the higher frequency part is lost. Ideally this would be with little undershoot and not too much slow decay at the very end. However nearly every low pass filter will do pulse stretching and the differences are not that large. Even a not so good solution would still be linear. Optimization is a little about the last little bit of noise / sensitivity.

--- End quote ---

This doesn't make sense to me.

The part the OP is interested in (I believe) is the rising edge and the initial peak.

If you apply a low pass filter to that, you will flatten the peak and get the wrong value. Totally defeats the purpose?

We want the information in the very steep edge. That has the highest frequency content. So why would we use a low pass filter? It will just give us some kind of "low frequency average"..which is exactly what we do not want.

You are right that "fast peak detect" is tricky. That's exactly because it has to deal with that high frequency content. Anyone can apply a low pass filter and get an average. Then I can get grandma to watch the value at the output of the low pass filter, and she will get it spot on?

snx:

--- Quote from: oschonrock on August 10, 2020, 12:47:51 pm ---Some clarifications:

1. I think "reshaping the signal" is a distraction. Any circuit which can "reshape it", can measure the peak (in analog domain or digital). Unless you call "peak detect" a reshape.

2. From what I understand, you just need the peak value right? You don't care about the exact shape?

3. What sort of accuracy are you after for the peak value?  You talk about 16bit. That's pretty accurate? Is that really necessary or will 12bit do? This is important because, together with the speed of the pulse, this will totally drive the solution. To get 16 bit accuracy, you don't just need a 16 bit ADC, your circuit and the power supply to it needs to be super noise free and you need a precision voltage reference (independent, the one in the CPU will almost certainly not do for proper 16bit accuracy). Your PCB will need to have fully seperate analog and digital sections with independent power supplies, independent ground/power planes etc etc etc.  So, will 12 bit do?  Note, that the scope you are using is just 8-bit....

As already discussed, either a fast ADC or an analog signal conditioner, followed by a simple ADC are the best bet. My instinct leans toward the analog signal conditioner first, then you can use CPU ADC. That LTC6244 App note (current boost option) is almost exactly what you need and for $10 BOM (2 "special" op-amps and a bunch of passives) and near zero design time, this seems like an attractive option. If it can do the job...

Also the discussion about amplifier gains is a bit premature for me. These are not super small signals. The small pulse has ~400mV peak. How accurately do you want to measure that small peak? To the nearest 5mV, which would be ~1% (8-bit is sufficient, if per-amplified or 12bit to give 16x dynamic range which seems more than enough for your pulse size range)? Or is 16 bit accuracy (0.0015%!!) necessary? If so then you are dealing with measuring and/or preamplifying a delta V of 400/65000 ~ 6uV. I am not even sure that's possible at this bandwidth...

But we need more data....

The pulse is "fast ish" but it's not clear to me from the traces how fast it really is. Can you provide some traces with the horizontal zoomed out to 2us? (ie 10x what it is now) And/or turn on a rise time measurement on the scope. We need to see that edge. The width is not important, it is the rise time that matters,
Thanks

--- End quote ---

Yes, correct the shape of the pulse is irrelevant, only the peak value is of interest.The peak is linked to the amount of radiation energy hitting the detector, the higher the energy, the higher the amplitude will be. In terms of Resolution, i need 4096 Channels, so basically a *perfect* 12-bit ADC, so in reality maybe a 14bit. Sure the scope has less, but usually scopes are to visualize values, not exactly measure them, that's why you use a bench top multi meter to measure voltage and not just the DC-Meter in the scope.  Also, the signal can be reshaped or transformed freely, unless the output has a defined link to the input. I've also seen approaches to measure the peak by simply use a Comparator to create a rectangular pulse, and then precisely measure the "on" time with a FPGA, but that's. The pulse length is linked to the pulse height, so this works. But that's another approach.

Detectors are limited towards energy depending on size, so lets say for this one, the maximum possible value out of 4096 Channels is 3000 Channels, the count rate above 3000chnl is so low, it can be ignored. Tricky is the thing, that the gain has to be set-able, because for example at low bias voltage, the 4069 Channels are spread between 0 and 400mV for example, but at high bias they are spread between 0 and 1200mV. If its unity gain 1, the problem would be that if we use a 10bit ADC for example,  when the value at high bias would be 0-1000mV, we would have 1024Channels, but if the bias is low (0-500mV) we only have half of values. The further processing of data would be to sample a collection of peaks, for example 5000pcs. Then a visible peaks in the values should be visible. For example, a calibration source would put two peaks in the spectrum, one at 32keV, and the other one at 661keV.  These are linked to ADC samples in case of spacing, so we would know between the adc reading x and y, where x = 32keV and y=661keV, there are 629Channels, so all values that not mach a channel would be rounded to the next one.


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

Marco:

--- Quote from: snx on August 09, 2020, 06:37:26 pm ---i would like to shape this pulse to a more Gaussian shape with uniform rise and fall times (like sine-wave).

--- End quote ---

Meh, don't make things harder than they need to be ... just R-C filter it, then a PGA.

Marco:

--- Quote from: oschonrock on August 10, 2020, 03:20:55 pm ---The part the OP is interested in (I believe) is the rising edge and the initial peak.

If you apply a low pass filter to that, you will flatten the peak and get the wrong value.

--- End quote ---

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.

snx:
By the area of the curve you mean like sample and then geometrically restore the peak by adapting the measurements to a known geometrical shape? Like measuring a sine wave with a low bit resolution?
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? Would it not even be more vital to shape the output to a better shape?

Also, i noticed that for example Amptek has a similar pulse shown here:
https://www.amptek.com/products/digital-pulse-processors/dp5-digital-pulse-processor-and-mca

(2- Prefilter Output - ADC Input

looks really similar to my pulses, but they have 80MSPS 12bit ADC which is extremely faster than my 2MSP. They would get 800 Samples in 10us

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