Re: FDC/CDC ASIC meeting minutes

[Gerard Visser <gvisser@indiana.edu>]

Hi guys,
	Yves, can you clarify, the gains listed in table 2 are determined from 
the scope photos shown in fig. 8, right? Where exactly on the histogram 
for instance in 8(b) did you call the peak amplitude? It seems to me 
rather subjective, and perhaps there is no reason to expect this 
procedure to asymptote to a constant gain as you lower the bias voltage. 
I agree with your conclusion that the plainly evident change of preamp 
pulse shape at higher bias voltages affects the effective gain. Is it 
not possible that the shape continues to change at lower voltages? (You 
could capture a set of waveforms, normalize _each_ individually to same 
peak amplitude, and then average them, and compare the results of this 
procedure at different bias voltages. This could provide some 
quantitative indication of how much the shape is changing from 
saturation of the preamp or whatever other effects.) Finally, one other 
point, the scope trigger threshold in fig 8 is pretty high (compared to 
the noise level), it seems. This must have some effect on the effective 
gain that you get from reading these histograms. Can you try to repeat 
at much lower threshold and see if it gives better results?

	- Gerard

p.s. Are the histograms (I presume in a narrow time window) centered on 
the peak time of the pulse in each of the 6 cases independently, or at 
fixed time, not therefore picking up the peak amplitude? I presume/hope 
you have done the former.

p.p.s. One more thing, the double-humped spectra, did you guys discuss 
could it have anything to do with the double-humped spectra you see from 
the preamp out at higher bias voltages in fig 8? Therein, it looks like 
some feature saturation behaviour of the preamp ASIC...


Simon Taylor wrote:
> Hello.
> 
> Fernando, Yves, and I had a meeting on Wednesday to discuss the FDC and
> CDC ASIC documents.  The minutes of this discussion are posted on the
> GlueX wiki at
> http://www.jlab.org/Hall-D/software/wiki/index.php/CDC/FDC_ASIC_Discussion_Meeting%2C_September_3%2C_2008
> 
> Simon