[Halld-tracking-hw] fADC range (was FDC meeting minutes...)

[Gerard Visser]

Hi Fernando,
	I agree completely it should be specified in terms of real pulses... But which 
ones? The variability of the pulse shape precludes a simple statement like "the 
full scale input range of the ADC is xx mV for a CDC pulse". It just doesn't 
work like that. This point seems to cause some confusion.
	I think in practice we should evaluate the suitability of the existing gain and 
shaping, change the shaping if that is indicated, _then_ by consideration of 
pulse height histograms for (as) real (as possible) detector signals decide to 
change the overall gain by a numerical factor to keep the fraction of clipped 
pulses to an acceptable level. This avoids the need to specify the input range 
in mV which is not simple.
	Nevertheless, if we prefer the other approach I can make that work too, but we 
have to be very specific (i.e. send me a datafile (measured or simulated) to use 
with my arb generator) about the input waveform which is to be scaled to full 
scale response on the ADC. It doesn't suffice to say so many mV full scale.
	Sincerely,

		Gerard

p.s. Of course we (I think) have to recognize that the pulse shape will be 
different in the presence of the magnetic field, so how confident are we that we 
will know it? I think the cluster arrival time distribution can only become more 
spread out, though, so that would mean that the optimal gain of the ADC would 
increase for pulses with the magnetic field, we would not be in danger of having 
too much gain and clipped pulses. Only too little gain and lost dynamic range. 
Is this correct?

Fernando J. Barbosa wrote:
> Hi Gerard,
> 
> I missed your email before I sent a reply to Naomi. My suggestion is to
> have the input range specified for real pulses out of the detectors,
> through the ASIC and 18 m of cable, add some margin and then use the
> fADC for validation. This is for one fADC assembly variant ("part
> number"). However, you may have a better plan to get the required
> information from the FDC and the CDC groups.
> 
> I should remind everyone that our schedule shows production of the
> fADC125s starting in January. So, we need to work diligently on this.
> 
> Best regards,
> Fernando