Mystery scientific preamplifier - Reverse engineering - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 15, 2020, 03:29:12 pm
content: 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

author: Kleinstein
author_rank: Super Contributor
timestamp: February 15, 2020, 04:15:25 pm
content: 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.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 15, 2020, 04:24:31 pm
content: 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 15, 2020, 06:48:54 pm
content: 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.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 15, 2020, 11:00:41 pm
content: 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

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 15, 2020, 11:06:54 pm
content: Actually, shorting the 2M resistor in the feedback loop would probably do the trick.When you have a FET in the feedback loop of an op amp like this, is all the gain calculatins the same as for a regular op amp?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 16, 2020, 09:35:06 am
content: The FET stage adds additional gain to the OP. The calculation of the external gain would be the same as with a normal OP. However it requires some care to keep the overall amplifier stable, especially at low external gain. Chances are the OP with the extra FETs is not as well behaved as the usual OPs, more like one compensated for high gain only (e.g. OP37 oder LF357).Today there is less need for such a circuit as there are quite good FET input OPs available. It could still make sense for very low input bias.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 16, 2020, 11:19:13 am
content: To be honest the only place I have seen the FET modified op amp scheme is in electrometer amplifiers to get the leakage down to sub-femtoampere levels.I'll play around with it some more and see if I can get a stable unity gain transimpedance amp out of it!

author: David Hess
author_rank: Super Contributor
timestamp: February 16, 2020, 03:24:49 pm
content: Quote from: ChristofferB on February 15, 2020, 03:29:12 pmThat is right.QuotePin 8 is for adjusting the input offset.QuoteThe AC gain peaking in the T feedback network implies pulse counting.Quote from: ChristofferB on February 15, 2020, 04:24:31 pmYou can do that since the input divider ground the input with 100 ohms. The AD744 might not have had enough offset null range and it is usually a bad idea to use offset null to correct for external offset.Quote from: ChristofferB on February 15, 2020, 11:06:54 pmYes, the differential FET input stage only adds gain within the loop. The configuration shown is very standard where higher performance is required than an operational amplifier will provide. The feedback network from the output to pin 3 controls the overall response.The AD744 is an odd choice unless they needed higher slew rate than a bipolar part would provide. I suspect they picked it because it uses external compensation. I wonder if they original intended to only use the AD744 and changed it later by adding the discrete JFET input stage.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 16, 2020, 03:38:39 pm
content: Quote from: David Hess on February 16, 2020, 03:24:49 pmMy though is more like starting with an BJT based OP (e.g. NE5534 was popular in combination with external JFETs) and than switched to the AD744 for higher speed / slew rate.

author: David Hess
author_rank: Super Contributor
timestamp: February 16, 2020, 04:29:30 pm
content: Quote from: Kleinstein on February 16, 2020, 03:38:39 pmThe AD744 actually has two advantages over a bipolar part in this circuit. The JFET input stage inherently roughly triples the slew rate because of lower transconductance, although bipolar operational amplifiers can use transconductance reduction to do the same thing, and the external compensation allows for decompensated operation.I suspect the AD744 included external compensation for use in high gain microphone amplifiers. That feature is very rare in any modern part; if anything, they usually support overcompensation.I am always looking for modern parts which support external compensation. It is too bad that the AD744 is so expensive.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 16, 2020, 08:56:51 pm
content: Thanks for all the info!I tried (reversibly) chopping away at the feedback network, since I really want all my shaping done later in my signal chain.At low gains (2M feedback resistor shorted or reduced to 50K) it oscillates. at around 250K the gain is significantly lower, to the level where all but the largest signals are within 0-5V range. See attached. In the picture the signal has also been run through my shaping amplifier, but they look similar out of the preamp itself.I'm unsure where the massive undershoot comes from. It was present before I modified anything too. Also unsure if it will be an issue in a multi-channel analyzer.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 16, 2020, 09:01:19 pm
content: Since I have three of these boards I'm also considering harvesting one of the nice opamps for a very bare-bones charge sensitive preamplifier, something like this:but running +-12V as supply. And maybe with a less exotic amp as buffer. Or without buffer entirely.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 16, 2020, 09:27:59 pm
content: For the amplifier one would not need such a high grade OP. For a first test even TL071 grade should be OK. It would make the pulsed a little wider, but that is about it. OPs with 10 MHz GBW (e.g. OPA172, OPA197) are relatively cheap these days.A really fast amplifier is mainly needed if one cares about the timing, e.g. for something like correlation. For the pulse hight the speed is normally set by the multi-channel analyzer, especially the peak shaper part.It depends on the implementation of the peak hight measurement weather the undershoot really matters, most version should be ok with undershoot.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 16, 2020, 11:58:16 pm
content: Good to know. I just though the whole point of having stuff like pole zero cancellation in a spectroscopy was to avoid this behavior.Also, this setup is NOT optimized for impedance. I've just been eyeballing it all the way through, and I think I've read that the PMT should have a T with a termination right at the output. I'll work it out.thanks for the interest and help!--Chris

author: Kleinstein
author_rank: Super Contributor
timestamp: February 17, 2020, 09:54:45 am
content: I don't think a BNC T and (50Ohms) termination at the PMT is not the right way. If planed to go that way with 50Ohms at the output, the resistor should be inside the case. The TIA might have some termination - 50 Ohm input impedance would not be that unusual for a TIA and if needed one could add a few 10 Ohms in series.It still depends on the needed speed. Just spectroscopy does not need super sharp peaks and high BW and thus no termination.For the optics it is nice of the peak forming does not include an undershoot. The peak high measurement usually does not make use of the negative part. So this part is kind of unused and would thus be lost energy / information. However the simple methods should not give a large error from the undershoot - more like a tiny bit more noise.