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Re: New Preamp Tests
Hello everyone,
I just have a few comments regarding the latest tests that Simon and
I performed with the new preamp on the FDC prototype:
Polarity: The polarity at the output of Gerard's modules (shaper boards)
is reversed relative to what we would expect. Because of this, we are
using the anode preamp card (A inputs) on the cathodes and the cathode
preamp card (B inputs) on the anode wires. It is understood that the
dynamic range is not well matched in this way as the A and B inputs have
been designed to handle the anode and cathode wires, respectively.
Simon, can then connect the outputs to the ADC. The polarity reversal
has to do with the order of the outputs going into the shaper board.
Gerard, can you comment on this? Sometime ago you mentioned that the
polarity could be selected on the shaper board but maybe you were
referring to the final board.
Channels: The preamp card has only 16-channels populated (2 ASICs).
These channels are 1-8 and 17-24.
Gain: The HV can be decreased to reduce clipping/saturation but the
efficiency plateau is already narrow. The clipping/saturation was
observed on the cathodes and I believe this is occurring on the shaper
board used with the cathodes which has a x5 gain, I believe. This may be
a result of the higher gain on the ASICs (3.6 mV/fC instead of 2 mV/fC).
Grounds: The CDC prototype is quite larger than the FDC prototype and it
will be a good test vehicle for grounding optimization and
implementation on both the CDC and the FDC. I should also mention that
the grounding on the FDC prototype can be improved but any changes, for
the better, will be hard to see given the very low noise levels now. For
instance, floating the LV supply (instead of grounding at the supply)
changed the DC level at the output of the shaper by about 2 mV but
didn't change the noise ( still about 20 mV p-p). Although this change
is small, it is something to consider when setting the dynamic range of
the shaper.
I think we have a good plan to work out the details in parallel
early next year:
1. FDC: Once the shaping is properly optimized, Simon will get data with
the new preamps + cable + shaper and check the results against his tests
with the older preamp. He can also use the fADC250 at 125 MSPS to get
data for fitting the leading edge and obtain timing information. I
believe the CDC needs something like 2 ns or better.
2. CDC: Gerard will be visiting CMU and will help resolve the ground
issues. Data can then be obtained in a fashion similar to the FDC.
Eventually, there will be a need to test the system with the fADCs and
we need to start planning for that (VME64x crates, CODA3, etc).
3. Preamp, cable, etc: Tests will be performed on the preamp card, cable
, supplies, etc. so we can qualify the overall design by early March.
This is absolutely necessary for Mitch has to incorporate any changes to
the specifications of the ASIC into the GAS-2 prototype - fab submission
in early April.
I will update the qualification checklist with details and
specifications that need to be checked.
Regards,
Fernando
Fernando J. Barbosa wrote:
> Hi Yves,
>
> I looked at your most recent tests and the pictures really helped.
> It seems to me that all point to a ground problem - not easy to
> diagnose remotely. In particular, connecting the shield to GND or the
> bypassed HV causes oscillations is suggestive of noise being coupled
> from GND to the preamp input. I also noticed that there may be a long
> path from the end-plate (straw tube ground) to the preamp GND,
> although this is something to consider later and not the cause of the
> oscillations at this time.
>
> I have a bit of great news from here. Simon and I got the new
> preamps installed and running on the FDC prototype and the system is
> very quite (~20 mV p-p noise floor) and with huge pulses (some >1.3V).
> We looked at both anodes and cathodes with 18 m cables connected to
> Gerard's shaper cards (these should not be a factor in the
> oscillations you get, I think). Aside from a few things to check with
> Gerard (polarity and gain), the system looks good.
>
> Simon will send an email tomorrow to a link where all the pictures
> and notes will be placed. I suggest looking at the overall ground
> connections - a simple diagram illustrating every connection (shields,
> cables, braids, power supplies) would be very helpful. Per your
> suggestion, it would be great if Gerard could visit with you after the
> collaboration meeting. If you are here for the collaboration, we could
> also discuss this issue and go over the FDC set-up as a reference.
>
> Best regards,
> Fernando
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