Electronics > Projects, Designs, and Technical Stuff
Help with opamp for shaping circuit
OM222O:
I would suggest you find a general equation for the decay of your signal, if you decide to go for the pure sampling method, that way you can work your way back and reconstruct the actual peak since it's likely you won't catch it exactly.
regarding digital pots and programmable gain amplifiers, the PGA inside the op amp is basically the same thing. It's a very wildly used circuit and I'm not sure what your problems are with it? the delay in changing the setting? if so that doesn't cause any issues since OP mentioned he doesn't need dynamic fast switching, just adjustment before each test.
oschonrock:
--- Quote from: snx on August 10, 2020, 11:14:39 am ---So you think the ADC of the MCU can be used for this?
--- End quote ---
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 pre-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
rstofer:
Dave did a video that winds up with an op amp precision peak detector (the 3rd circuit iteration), maybe it is useful...
Kleinstein:
The analog peak detector gets tricky if it has to be fast and accurate at the same time. So one would need peak stretching anyway. Even than 10 bit accuracy gets tricky and slow.
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.
The LTC6910 and LTC6088 look good. The LTC6268 is more like overkill and the ADA4622 may not be happy with well below 5 V supply.
For looking at the peaks I would prefer looking at the area instead of the actual maximum value. With scintillators both should be proportional to the energy as the peak shape should be fixed. Chances are the area can be more stable (e.g. with temperature). In addition the area measurement can be lower noise as it uses more than just 1 ADC conversion. With enough computing power the ideal way would be a kind of peak fitting: with parameters amplitude and time. This is kind of in between the peak hight and area. With enough simplification this may be a feasible way: calculate the correlation with 2 test shapes (about the expected shape to get the amplitude and one closer to the derivative to fix the position).
There is also a semi analog way to the area way: have an extreme low pass filter close to an integrator with a very slow reset and measure the quasi DC value before and after the peak.
I think the ADC inside the µC can be OK if the µC gets a clean supply (e.g. LDO after SMPS). However I have no experience with the ST µCs and don't know how sensitive it is to the actual layout.
This is because I very much doubt one would really need perfect 12 or 13 bits. Only really high end detectors (e.g. cooled germanium direct semiconductor detector) have such a resolution. For the gain error the more problematic point is a stable detector gain.
The OPs would likely be supplied by the same 3.3 V or similar voltage as the analog part of the µC.
oschonrock:
--- Quote from: rstofer on August 10, 2020, 02:17:27 pm ---Dave did a video that winds up with an op amp precision peak detector (the 3rd circuit iteration), maybe it is useful...
--- End quote ---
Yeah good. I was going to find the link to that. This is a goo introduction. However, to be clear for the OP, even the final version of what Dave talks about is not at the level of complexity or performance of that ADI article linked to above.
Depending on the rise time of the pulse (waiting for detail) that ADI circuit, which catches the peak of a 200kHz sine wave as shown, may or may not be enough for this application.
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