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Design and PCB plan

A project log for Chymes

Cheap, nondestructive atomic analysis in real time.

A. M. AitkenA. M. Aitken 05/29/2016 at 19:360 Comments

In order to get decent performance I plan to build and then tweak several preamp and shaper designs. Some of these are simple and obvious but may need high quality components, some are very clever by the best in the field that publish and some will be crayon drawings by myself that lean on more expensive parts. The goal is to look at the trade off for the final design, performance versus cost.

At it's current state of development this project is probably a best fit for "Citizen science". I was hoping to have a proper shot at "Anything goes" but although I've read a lot, ordered some parts, daydreamed and dug out long forgotten priceless junk, I've so far accomplished nothing. Even putting some ongoing IRL issues aside I'm disappointed. In order to have any chance of being noticed by anyone, either to progress or just interested hackers, I feel I need to make very substantial progress by July 11th.

This must shape my plan. I don't have the time to evolve a design by international post and I don't need 10 copies of the same PCB to throw away the moment I have to make major track changes. Local small order PCB manufacturers work out many fold more expensive. I also need a large number of different simple circuits that with the addition of a few bus bars and copper foil shields would probably fit pretty well on a single side. I've dug out my UV exposure box and after 10 years gap hope to remember enough to do more than stain my fingers.

The plan right now is,

1. Fab preamp PCBs and test these with a second hand commercial shaper. The main advantage is that the shaping time can be changed over 2 orders of magnitude at the turn of a switch. The home made version will be much less flexible.

Fab the shaper op amp circuits at the same time. AD8012 may be a poor choice, the spec on paper is superb but being a current feedback amplifier brings a set of golden handcuffs and a mean attitude to the party. In addition Analog does not seem to be marketing the double CFB AD8012 among it's op amp line, even though the (slightly better specced) single CFB AD8011 is. I also worry that even though these should be fast and low noise that the preamp circuits are used for speed at the expense of energy resolution.

So also use a conventional design slow VFB op amp shaper. I'm wondering if a preamp with a slow op amp might be a better source for this circuit. CR-RC shaper may be fine and extra SNR can probably be recovered with processing. Probably best implemented as Differentiate-Amplify-Integrate (This is more a note to myself and will expand on it later).

2. With a working preamp and commercial shaper write the STM32 code to do Pulse Height Analysis that is the raw data for detection. Tweak for performance if needed. Test combinations. Abandon and move to an op amp based Wilkinson converter if the STM32 internal ADC isn't up to the job.

3. With a working preamp and home made shaper put everything together. Tweak. If the design had to move to a Wilkinson converter then pulse processing code is not possible and we need to move to a better shaper like CR-(RC)4.

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