Electronics > Projects, Designs, and Technical Stuff

Help with opamp for shaping circuit

(1/8) > >>

snx:
I want to sample and process a pulse from a Silicone-Photomultiplier.

The Pulse has amplitudes between 10mV and 2.5V (depending on light strength and bias voltage. Therefore, an amplification with switchable gain would be very helpful. For example if a high bias voltage is used, the gain can be zero, and when lower voltage for bias is used, the gain can be 2) The pulse length is from around 10us to 80us in a sharply rising and then slowly decaying curve, making it very hard to sample since the peak is of interest, and would have to be sampled very quickly after the trigger condition.

I've attached 3 Scope images of the pulses, one with persistence at the highest  bias voltage (Gain 0). And two were you can see a very large pulse (2.4V peak) and a regular pulse (400mV Pulse).

My goal is to sample this with a 16-bit 3.6MSPS or 2MSPS ADC (2.5V Reference IC) that is built in in STM32H750VB Microcontroller. I've attached the ADC specifications extracted from the datasheet for you. To have a good number of samples, i would like to shape this pulse to a more Gaussian shape with uniform rise and fall times (like sine-wave). Also i would be good to have a pulse length of around 15-20us so a 2MSPS ADC would get 30-40 Samples.

The MCU has two build in OP Amp, maybe these can be put to use, the specifications are attached. The Voltages currently planned to be available on the design are 5V from USB and 3.3V from SMPS, surely, if its vital to have other voltages, i can add them.

So i'm not that familiar with opamps, but 've seen designs with variable gains by switching resistors with analog switches. But i'm more worried what i would need to create the desired shape of the signal.

Can someone give me advice on a circuit for shaping the signal?
Do i need a OP-Amp for the ADC to buffer? (seen this on some adc datasheets).
Can the OP-Amp from the MCU be used, or are they too bad?

OM222O:
do not use an analog switch with many different resistors.

It seems like the peak is actually negative, then you most likely need an inverting amplifier setup. I strongly suggest you ditch the built in ADC / OP AMP since they're not actually optimized for anything, they're more for general purpose applications where accuracy is not that important.It all depends on what you need to do.

Use a dual pot, you can easily get 8 bit 10k digital pots for around 1$. it allows you to fine tune your gain. The absolute value of digital pots is really bad, but the ratio-metric arrangement is very accurate. Especially since they're on the same die, effects like temperature drift and whatnot won't change the ratio accuracy.

Try to set them to max values for any given ratio as they have a current limit, just something to be aware of.it's also easy to do the math as you can easily just use values from 0 to 255 (or 256, depending on the exact part you use, some use a 16 bit number and go from 0 to 256 instead of 0 to 255 or 1 to 256, just check the datasheet).

snx:
Good Idea with the digital potentiometer, i will try to implement this!

Any ideas on the OpAmps?

I need to extract the peak value of the peak. If the internal ADC of the MCU is capable, i would like to use it, since it saves cost an board space, but of course if its not accurate enough, i will  have to ditch it. The Problem is, that usually, when using high-bit, fast speed adc, you need to take care of signal routing and have high clock lines. For example a 3.6MSPS 16bit ADC needs at last 57.6MHz of Clock and probably SPI. Other high speed ADC's then to use LVDS, which is not supported by this mcu.

I've looked for standalone ADC with following parameters

16-bit
>3MSPS
1 CH
Serial SPI or Paralell Interface
For example, Analog Devices only hase 5 matches, all super expensive

Would LTC2311-16 (13.80$) https://www.analog.com/en/products/ltc2311-16.html be a much better choice than built in ADC?


Sure, the op-amp can be ditched, i just mentioned that its there  :)

OM222O:
the built in ADC seems to only go upto 3.6V max (3.3V normal operation) but you mentioned running on 5V? I'm not sure how that'd work.
Also there is no mention of INL + DNL or ENOB BUT there is a table at the end regarding RAIN( \$\Omega\$) which I'm not sure what input resistance that is exactly. Maybe you can make sense of that and how accurate it is?

I personally use sigma delta ADCs with only a few KPSP but 24 bits high accuracy, since I don't have fast signal applications, you should look into SAR ADCs for fast sampling if you want to buy an external one. Unfortunately I can't really help you with that. As for the Op Amp: how much gain bandwidth product do you need? any ideas on the min/max required gains? 3 good suggestions would be MAX4238 (unity gain stable), MAX4239(at least a gain of 10), INA189. I have used these for high accuracy measurements before and they work like a charm. Actually Dave recommended all of them in a recent video which was surprising, each of them has some benefits over the others so take your pick between them.

Edit: my bad, you mentioned 2.5V not 5V. please ignore that comment.

Kleinstein:
16 bit resolution sounds plenty. With so much resolution there is no need to switch the gain, at least not in fine steps. Digital pots tend to be relatively slow with quite some unwanted internal capacitance. In addition the fet switches can act nonlinear. I would consider adjusting the high voltage to trim the gain if needed.

AFAIK the energy resolution is often quite limited, so more like 10 or 12 Bit. This is especially true for the more low cost type detector.

For the detection of the peak there are 2 options: either the maximum value, or the area under the curve.

For the OP I would expect something like a relatively fast CMOS OP, maybe LTC6240 or similar of price is not that important. The baseline (DC level) can probably be subtracted in software.

An important factor would be the expected count rate. The more the less pulse stretching is possibly without loosing to much to pulse collision.

In principle the pulse stretching would be similar to an anti aliasing filter - the rest would in theory be done in math.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod