DIY Charge sensitive Amplifier

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Lukas Springer
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DIY Charge sensitive Amplifier

Post by Lukas Springer » Sun May 19, 2019 8:25 pm

Hello!

Since my HP 5554A died I was in need of a proper charge sensitive amplifier for my detectors.
They're not often on ebay, and if they are it's $100+...

So I designed my own:
The Operational amplifier is the heart of my design, that's why I choose a very fast one, with very low input current.
The LTC-6269-10 is a dual opamp with 4 GHz and 3 fA input bias current, which makes it a perfect choice, only the supply and therefore output voltage range is somewhat low, limiting the amplifiers dynamic range.
Capture.PNG
The amplifier settings result in 300, 870, 1460 and 2560 mV/pC.

It works well with Semiconductor Detectors (tested with several Ion implanted Si detectors) and proportional detectors (tested with several helium-3 detectors), but I did not yet measure noise figures.

This is the output pulse with 2 pC input:
DS1Z_QuickPrint4.png
DS1Z_QuickPrint4.png (36.04 KiB) Viewed 3589 times
this is the output pulse with 100 fC input:
DS1Z_QuickPrint5.png
DS1Z_QuickPrint5.png (36.14 KiB) Viewed 3589 times
The Amplifier has to be well shielded and the input has to be very low capacitance, otherwhise you'll get tonnes of noise and or no signal.
photo_2019-05-19_22-22-49.jpg
photo_2019-05-19_22-22-52.jpg
If you have any questions feel free to ask

Lukas

Lukas Springer
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Re: DIY Charge sensitive Amplifier

Post by Lukas Springer » Mon May 20, 2019 10:04 pm

Did some noise measurements, noise is equivalent to 2.5 fC / 115 keV (Si).
Next revision will half that, a smaller feedback cap should also fix this.

John Futter
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Re: DIY Charge sensitive Amplifier

Post by John Futter » Tue May 21, 2019 4:13 am

Lukas
To obtain what you want you will have to aerial pin 2 neg input to opamp to get lower capacitance and the required resistance ie the 22meg gets shunted by pcb surface leakage
That is why when as a Junior electronics engineer many years ago we could not use a resistor over 2.2 megs. If you wanted a higher resistance you had to series them up. All of this so we kept full spec from -30 dgrees celsius to + 60 degrees 95% relative humidity non condensing

Lukas Springer
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Re: DIY Charge sensitive Amplifier

Post by Lukas Springer » Tue May 21, 2019 8:27 am

The pin does not have to be lifted, with proper guard traces the capacitance can be held at a minimum.
Also, this is a CSA, not a voltage amplifier, the input capacitance is not as important, since it only affects pulse shape, not the absolute output.
The parasitic leakage and capacitance of the resistor in the feedback network can be lowered to a minimum using a grounded guard trace below the resistor. The Integrators time constant checks out with 2 pF / 22 MOhm.
Please read the Datasheet of my opamp, it's got some usefull tips on how to lay out PCBs without having to lift every other pin off the PCB.
If you are interested in further reading on readout electronics I recommend this paper:
http://www-physics.lbl.gov/~spieler/ICF ... ectors.pdf

Lukas

John Futter
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Re: DIY Charge sensitive Amplifier

Post by John Futter » Tue May 21, 2019 10:11 am

Lukas
I'm quite ok that this is a charge sensitive amplifier
and stray cap on the input = noise
I didnt5 realise that you were using PTFE PCB as G10 specs are different from your situation

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Richard Hull
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Re: DIY Charge sensitive Amplifier

Post by Richard Hull » Tue May 21, 2019 7:17 pm

I have always lifted key pins as I have made many electrometers using fempto-amp bias ICs and the venerable 5886 which is easy to clean to retain spec. A foam pad is needed with vacuum tubes to prevent microphonics. I rarely use the 5886 in any new effort due to the time it takes to reach some degree of stability. Only in lightning and HV static experiments do I resort to toobs, (glowFETs).

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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