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[Gerard Visser <gvisser AT indiana.edu>]

hi Xu, Zhenyu,
	Thanks; I didn't realize about this timebin dependence in the IST. By the way 
do we see any dependence of noise on the "cap id" (i.e. the "address" reported 
in the APV header)? I am assuming we are triggering at a low rate and 
asynchronous to the clock (ARC-II local clock) so we should be getting data in 
all values of capid. This could show some features (and perhaps can allow for 
some capid-dependent correction applied offline).

	I make some estimate of the capacitances as follows: Neglecting the capacitance 
to neighbor pads, and all the capacitance of the routing line on the detector, 
let's consider only the infinite-parallel-plate capacitance in bulk of the detector:

IST: Pad size is 594 x 6275 um, thickness 300 um, k=11.7  ==> C=1.3 pF (I think 
this must certainly then be dominated by the other, neglected capacitances 
mentioned above).

FST: worst case outer pad size is (about) 1087 x 28750 um. (Right? If you have 
more precise info please say.) Also 300 um thick. ==> C=10.8 pF. (+ other again)

	If you have some info about the gap in contact/metal between adjacent pads in 
the case of IST and FST I could try to roughly estimate the perimeter 
capacitance to neighbor pads.
	If you have some measured capacitance info or real calculated capacitances from 
detector design, of course we could better think about those.
	Anyway, my guess is that the capacitances in FST (outer at least) are probably 
2-3x the capacitances in IST. This is probably responsible for the larger common 
mode noise and larger noise. If so unfortunately it probably means there is 
nothing that we can do about it.
	If we have an IST stave with a defective detector and wanted to do further 
tests, we could bond some APV input pads to test capacitors of value similar to 
the FST detector and see how that looks. I think this could be a significant 
effort though.
	Really the only question that we must answer is whether the PPB, purple cable, 
T-board, and hybrids are working well together to deliver clean supply voltages 
and clock/trigger signals to the APV chips (and clean bias to the sensors). This 
is probably so, I think all your noise plots look reasonable, but we could try 
to check more directly with low noise probing of the supply voltages on one of 
the prototype hybrids. We probably would have done this already if it weren't 
for the virus situation.
        Sincerely,

                Gerard


p.s. I suppose that, at least for lower trigger rates <3 kHz or so, we should 
consider to setup to read 4 timebins and ignore 0 in offline, use only 1-3. If 3 
timebins is all that we really need, that is. What do you think?


On 8/10/2020 2:29 PM, Xu Sun wrote:
> Hi Gerard,
>
> Sorry for the late reply. Please find the FST & IST noise study in the attached 
> file.
> I see a similar behaviour for IST with a much smaller magnitude.
>
> Best,
>
> Xu
>
> On Tue, Aug 4, 2020 at 1:12 PM Gerard Visser <gvisser AT indiana.edu 
> <mailto:gvisser AT indiana.edu>> wrote:
>
>     Hi Xu, Zhenyu,
>              Do we see the timebin-dependece of noise as you show here
>
>     
> https://drupal.star.bnl.gov/STAR/event/2020/05/04/star-forward-silicon-tracker-meeting/prototype-module-assembly-and-test
>
>     in the IST data too?
>              Thanks,
>
>                      Gerard
>
>
>     p.s. And, if possible to answer, there is also the question whether this 
> was
>     seen in the IST installed in STAR/HFT? I don't remember hearing about it 
> before.
>
>
>     On 8/4/2020 12:39 PM, Zhenyu Ye wrote:
>      > Hi Gerard,
>      >
>      >> On Aug 4, 2020, at 11:19 AM, Gerard Visser <gvisser AT indiana.edu
>     <mailto:gvisser AT indiana.edu>> wrote:
>      >>
>      >> hi Zhenyu,
>      >>      That timebon dependence sounds definitely odd. Are you sure? Do 
> we
>     see that in IST too, only more mildly? I wasn't aware of this.
>      > ‘
>      > Please take a look at Xu’s presentation in FST meeting on May 4 (FST) 
> and
>     11 (FST and IST):
>      >
>      >
>     
> https://drupal.star.bnl.gov/STAR/event/2020/05/04/star-forward-silicon-tracker-meeting/prototype-module-assembly-and-test
>      >
>     
> https://drupal.star.bnl.gov/STAR/event/2020/05/11/star-forward-silicon-tracker-meeting/prototype-module-assembly-and-test
>      >
>      >>      The capacitance of FST is much larger than IST, I think. This may
>     certainly be relevant. For sure it is relevant to CMN.
>      >>      Anyway, I agree we should investigate these noise issues, I do 
> not
>     like to ignore them. However on the other hand it _may_ be an inherent
>     property of APV chips.
>      >
>      >>      - Gerard
>      >>
>      >>
>      >>
>      >> On 8/4/2020 12:12 PM, Zhenyu Ye wrote:
>      >>> Hi Gerard,
>      >>>  From the plots that Xu sent, there is a clear pattern where the
>     channels showing enhanced noise level after mounting the sensors, also 
> show
>     enhanced noise level before mounting the sensors.
>      >>> An independent topic, we see that the CMN in FST is much higher than
>     IST, and show a strong time-bin dependence, i.e., when we read in 9 time
>     bins, the 1st, 5th and 9th time bins have much higher CMN than the other
>     time bins. I don’t feel comfortable to ignore it w/o knowing the cause, as
>     it may get worse in the real experiment.
>      >>> Best,
>      >>> Zhenyu
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