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Mystery scientific preamplifier - Reverse engineering

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ChristofferB:
Hey, This is a somewhat tangent to my photomultiplier thread but pretty distinct.

I have a bunch of preamplifier boards from PMT's, which are constructed as little DIN41612 plugin boards. No info beyond that.

They are really nice, though, with an exotic dual FET input stage and a nice AD744 op amp.

I'd like to reuse them for my scintillation/cherenkov counting adventures, however their function puzzles me somewhat.

I've traced the schematic from the pcb, and it looks like they have +-V, GND, out, and 2 input pins.

Now, from the feedback stuff and BAV45 "picoampere" diodes and feedback network, I believe pin 16 must be main input,

That just leaves pin 8 as an unknown input. Can anyone deduce what its function is? Is it just some programmable offset thing?



I'm not deep enough into these circuits to deduce whether this is optimized for pulse counting (single photon, what I'm looking for) or some continuous light level thing, what do you think?

The two unknown components looks like laser trimmed resistors, but unfortunately I don't have my good multimeter with me so I'm unsure of their values.
Maybe thermistors for temp. co?


Thanks for the interest!

--Chris

Kleinstein:
The unknown white SMD parts could be just low value (e.g. NP0  type) ceramic capacitors.

The Circuit looks like some kind to trans-impedance amplifier and pin 8 looks like an offset adjustment.
The combination of rather high value resistors and a fast OP is relatively odd.

ChristofferB:
Thanks for the comments!

Since the "offset" pin is grounded through some resistors, could I leave it unconnected?

Maybe the best solution is to give it +-12V and see how it responds to input pulses.

I must admit, the very complex feedback net kinda puzzles me though.

Kleinstein:
The complicated feedback looks like a kind of "peak"- shaping or correction of the frequency response. I don't know if this amplifier is more suitable for a PMT, a photo-diode or maybe another sensor.
With the high resistors it looks a little slow for use with a PMT.

ChristofferB:
I just tried it out with +-12V and a BNC on in/out, hooked from PMT to scope, and it is definitely working! It has a decent amount of gain, but also some pulse shaping operation!

I tried tweaking both pots to determine what they do. one reduced the negative swing of the pulse signal, the other (10 turn) had no discernable reaction.

I have a few so I think I'll try removing the feedback net of one of them and just put in a 1K resistor and a 5pF capacitor in parallel to make a 'low gain' version

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