Hi Bob,

Sorry, I was away yesterday. I quoted a values for V(FET) of 0.25
nV/[root(Hz)] in the note I attached a few days ago, and a value for C(FET)
of 60 pF.

The noise I measured for the endcap feedback components (8p2//4k7 at an
added input capacitance of ~50 pF) was ~3300e. For 40 ns shaping the
feedback resistor should contribute 2236e, and the FET noise should be 976e
(for C(TOT) = 130pF), making a total prediction of 2439e. This is somewhat
less than the measurement, but there are some factors which are overlooked
in the simple calculation.For example, the pulse shaping is assumed to be
perfect, and no contributions from the MGPA first stage other than the
input FET are included, and no contributions from subsequent stages

 From experience, achieving 100% agreement between predicted (or
calculated) and measured noise is difficult. Measurements can be easily
affected by interference in the environment. I am confident that the lab
measurements are fairly accurate though, especially since the barrel values
have been pretty much confirmed in the supermodule test beam system.

I think the decision on endcap feedback components should be based on
dynamic range. The choice is probably between 8p2//4k7 or 6p8//5k6. I am
happy with either. The noise difference will be small, and I believe both
will meet the specification.

I will get the VFE boards I have modified soon after Christmas, so we
should probably decide by the end of the first working week in January (the
7th).

Cheers, Mark.

At 17:20 17/12/2004 +0000, Brown, RM (Bob)  - PPD wrote:
>Mark, thanks for this explanation.
>
>I am slowly getting my head round the various noise contributions.  I can
>more or less reproduce the estimated figures that you give in various talks
>etc, but I find some differences when I try and calculate the FET
>contribution.
>
>What value do you assume for V(FET)?
>
>I assume that I have to include C(FET) in C(Total), what value do you take
>for this?
>
>Bob.
>
>-----Original Message-----
>From: Mark Raymond [mailto:m.raymond@imperial.ac.uk]
>Sent: 15 December 2004 16:47
>To: Brown, RM (Bob) - PPD; claire.shepherd-themistocleous@rl.ac.uk; Lodge,
>AB (Tony); Bell, Ken; matthew.ryan@imperial.ac.uk
>Cc: Cockerill, DJA (David)
>Subject: RE: protection resistor noise
>
>
>Hi Bob,
>
>Decreasing the feedback capacitor increases the gain for both signal and
>series (FET) noise so the ENC contribution from the FET is unchanged.
>If  Cf is reduced then Rf has to be increased to maintain the 40 ns time
>constant. Increasing Rf leads to a reduced parallel noise contribution from
>this component.
>
>Of course the final noise is a combination of all contributions, so another
>way of looking at it is that as the Rf noise reduces, other components
>become more significant, but they don't actually increase. The bottom line
>is that the bigger you can make Rf, the better, but this does come at the
>cost of reduced dynamic range.
>
>The table you saw yesterday shows hand calculations of the benefit of
>increasing Rf. You can find it again at:
>
>   http://www.hep.ph.ic.ac.uk/~dmray/pptfiles/EEisues.ppt
>
>The measured noise for the 8p2//4k7 combination is also shown. The measured
>values are a bit larger than I estimated. It is possible that there was
>some interference contribution in the environment, although I took some
>care.
>
>Anyway, if we assume the noise is 3300e for Cf//Rf = 8p2//4k7, it should
>reduce to 3175e if we go to 6p8//5k6.
>
>I don't think we should go any lower, as we will become more sensitive to
>stray capacitance (in parallel with Cf) on the VFE card, which might lead
>to channel to channel pulse shape variations (because Cf determines the
>pulse shaping differentiation time constant).
>
>So, the noise benefit is rather small, but there is no point having more
>dynamic range than you really need.
>
>Hope this helps.
>
>Cheers, Mark.
>
>
>At 15:19 15/12/2004 +0000, Brown, RM (Bob)  - PPD wrote:
> >Mark, thanks for this.
> >
> >My current thinking is to reduce the dynamic range by decreasing C(f)
> >to 6.8pF, giving full scale = 10.5 pC. This is because the crystal and
> >VPT will 'age' reducing the charge/MeV with time.  At the start we can
> >reduce the VPT bias to avoid saturation, then increase the bias later
> >to compensate for aging.
> >
> >However, I now realise that although decreasing the MGPA full scale
> >decreases the R(f) noise, this improvement is offset by an increase in
> >the FET noise, which starts to become important when C(f) becomes
> >small.
> >
> >Have I understood correctly?  What is your estimate for the total noise
> >with
> >C(f) = 6.8 pF and R(f) = 5.6k?
> >
> >Bob.
> >
> >-----Original Message-----
> >From: Mark Raymond [mailto:m.raymond@imperial.ac.uk]
> >Sent: 15 December 2004 13:20
> >To: claire.shepherd-themistocleous@rl.ac.uk; Lodge, AB (Tony); Bell,
> >Ken; matthew.ryan@imperial.ac.uk; Brown, RM (Bob) - PPD
> >Subject: protection resistor noise
> >
> >
> >Hi,
> >
> >I gave some consideration to the noise contribution of a possible
> >protection resistor and am attaching a note. I was too pessimistic
> >about its noise contribution as you can see, but it depends on where
> >you put it. If it can go on the HV card, close to the VPT then its
> >contribution will be small. If it goes on the motherboard and sees the
> >significant capacitance of the umbilical cable, then its contribution
> >will be much larger.
> >
> >I assume this accounts for Claire's measurements.
> >
> >Cheers, Mark.
> >
> >------------------------------------------------------------------
> >   Mark Raymond
> >   Blackett Laboratory
> >   Imperial College       phone: +20 7594 7799
> >   London SW7 2AZ         fax: +20 7823 8830
> >------------------------------------------------------------------
>
>------------------------------------------------------------------
>   Mark Raymond
>   Blackett Laboratory
>   Imperial College       phone: +20 7594 7799
>   London SW7 2AZ         fax: +20 7823 8830
>------------------------------------------------------------------

------------------------------------------------------------------
  Mark Raymond
  Blackett Laboratory
  Imperial College       phone: +20 7594 7799
  London SW7 2AZ         fax: +20 7823 8830
------------------------------------------------------------------