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[Gerard Visser <gvisser AT indiana.edu>]

hi Zhenyu,
	That may certainly be correct that the strip to strip capacitance will 
dominate. Let's try to estimate it. Can you say what is the width of the gap 
between strips (i.e. between contact regions or between metal, whichever gap is 
the smaller)?
	Of course the routing line capacitance also matters and I don't have any way to 
estimate that. It would be better to know the overall detector capacitances from 
a real calculation or measurement.
        Thanks,

                Gerard


p.s. Some mechanisms for common mode noise, such as noise in the bias or noise 
in the frontend amplifier power supply, will have a term proportional to 
detector capacitance. I'm sure it is true that other mechanisms exist that 
don't. Xu did send plots earlier that showed the CMN increasing significantly 
with detector capacitance, I think. It could be due to power supply noise and I 
agree completely this is something worth trying to measure and be sure there is 
no problem with our frontend design that should be resolved. I think Ben Buck 
did this noise measurement on IST hybrid at one point, but I don't have detailed 
info on that.


On 8/11/2020 5:28 PM, Zhenyu Ye wrote:
> Hi Gerard,
>
> The total capacitance of a readout strip for silicon strip detectors is given 
> by the sum of the strip-backplane capacitance, and the inter-strip 
> capacitance. The latter usually dominates.
>
> As we know, the APV noise is linearly dependent on input capacitance, i.e., 
> the total capacitance of the readout strip. For FST, we have seen that outer 
> (larger R, Rstrip3 for the inner sensor or Rstrip7 for the outer sensor in 
> Xu’s plots) strips have much smaller noises than the inner strips (Rstrip0 
> for the inner sensor or Rstrip4 for the outer sensor), despite the fact that 
> outer strips have larger area and thus large strip-backplane capacitance. 
> This is consistent with the above statement that the interstrip capacitance 
> dominates.
>
> The CMN, by definition, affects a group of channels in a coherent way. It is 
> usually caused by a common electromagnetic pick-up, or noise on the supply 
> voltage, etc. The fact that we see large CMN in FST, suggests to me that we 
> should check FST inner cable/T-board/hybrid, such as grounding.
>
> P.S. Xu will send some plots on FST prototype modules before sensors were 
> mounted. We see large CMN with strong time-bin dependence. I think this is 
> consistent with my above assessment.
>
> Best,
> Zhenyu
>
> > On Aug 10, 2020, at 3:45 PM, Gerard Visser <gvisser AT indiana.edu> wrote:
> >
> > 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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