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

[Fernando J. Barbosa]

Hi Gerard,

I think we have obtained a "library" of pulse shapes from the CDC and 
the FDC over the last few years to get an idea of their variability. I 
agree that optimization of the full scale range can only be attained 
with the real range of pulse shapes but a simple description of the 
input amplitude range is a good starting point and that's what we have. 
I think we both agree that we should proceed with tests with the 
existing fADC125 settings and see if we need to change anything later, 
from detector tests and from simulations with the magnetic field, etc.

Given your work required for Rev 2, which can proceed in parallel, the 
CDC and the FDC groups will need to complete their tests with a full 
readout chain by the end of November. We will decide on the final 
settings for the fADC125 at that time and in preparation for production 
in January. Note that there is not much time at the end of the year due 
to holiday closures.

Best regards,
Fernando



Gerard Visser wrote:
> 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
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