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Re: question on the CDC/ASIC doc
Hi Gerard -
I am still trying to finish that document, but wanted to get something on the
portal for today. With regard to the factor of (3) vs (2.2), I think we still need
to get that right. I want to wait for Yves to get back to have the final discussion
on that.
As per the gain, my understanding is that the peak amplitude is integrated over
som number of flash adc channels. As such, it has to be initially in mV, and then
converted back into a charge. This also makes me question the 15% factor associated
with the (3) or (2.2) above.
- Curtis
On Tue July 8 2008, Gerard Visser wrote:
> Hi Curtis, Yves,
> I'm reading the freshly posted document on your CDC/ASIC studies. _Now_
> I understand (I think) that the sharp cutoff in for instance fig 2a/2b
> are the trigger threshold. But then _what_ is all that stuff below
> threshold? Are you not triggering only on the channel of interest here?
> If you're not triggering only on the channel of interest, it would
> probably be interesting now to see the total charge (integral) spectrum
> when the trigger is changed to be only on the channel of interest. (For
> some reason you've left off the total charge spectrum from the document?)
> Also, I guess you should now remove the shaper "gain" of "3" stuff, we
> are all agreed now that it is 2.2. (For "instantaneous" charge input to
> the ASIC, with little or no detector capacitance; it may be fairly
> different for other situations possibly including the real CDC.)
> Thanks,
>
> Gerard
>
> p.s. Finally one more question, is the peak amplitude, such as plotted
> in fig 2, simply the highest sample point, or is it based on a fit to
> the waveform around the peak? In the former case, of course it suffers
> from the _fact_ that a sample point might be on the actual waveform
> peak, or the peak might straddle two sample points, this could make a
> 10-30% random error depending on the relative time of the peak and the
> sample clock. [This is another reason why it might be better to
> concentrate on the total charge spectrum instead of the peak voltage
> spectrum in figuring the gains. At least, in my opinion.]
>
--
Professor Curtis A. Meyer Department of Physics
Phone: (412) 268-2745 Carnegie Mellon University
Fax: (412) 681-0648 Pittsburgh PA 15213-3890
cmeyer@ernest.phys.cmu.edu http://www.curtismeyer.com/