[Halld-tracking-hw] FDC meeting minutes posted

[Fernando J. Barbosa]

Hi Gerard and Lubomir,

The peaking time with the preamp and 18 m of cable is 14 ns and so it is 
the minimum one can achieve. I agree that 25 ns would be the minimum in 
trying to get three samples on the leading edge for timing 
interpolation. On the other hand, a 35 ns peaking time is very 
reasonable so we are perhaps considering optimizing this between 25 ns 
and 35 ns and based on the benefits of noise shaping. I don't think 
Lubomir included the time walk correction but this can be found on 
GlueX-doc-1364 for the ASIC (GAS-II).

Regarding the assembly variants, I agree with Eugene that we should try 
to get one fADC125 but we need to consider this carefully. We have 
considered two fADC125 assembly versions, one for the CDC (low gain 
preamp) and the other for the FDC strips (high gain preamp) primarily 
based on fully using the ADC dynamic range. The preamp output ranges (@ 
saturation) are not the same in the configurations we are using: 430 mV 
for high gain and 330 mV for low gain. If we set the ADC full scale (FS) 
at 430 mV (4095 on 12-bit), then 330 mV will be at 76% FS (3100 on 
12-bit). Are we willing to sacrifice 25% of the ADC range on the CDC in 
exchange for one fADC125 version (based on saturation conditions)?

However, there is another issue to consider if we are to have a single 
ADC. The output saturation at 430 mV is way out of the linear range and 
the output is 285 mV @ 5% linearity. For the CDC and the FDC:

CDC - 330 mV @ Saturation, 207 mV @ 5% linearity
FDC Strips - 430 mV @ Saturation, 285 mV @ 5% linearity

For a single ADC solution, I propose we set the ADC FS at 380 mV. We can 
set this even lower for a tighter linear range of interest.

Best regards,
Fernando





Gerard Visser wrote:
> Hi Lubomir,
> 	We should perhaps discuss the shaping option ideas more fully in the next 
> meeting, I can call in. I didn't realize this was on the agenda today.
> 	In my opinion it is *feasible* to support two different versions of the module 
> with different shaping time. This amounts only to different values for some 
> capacitors, inductors, and resistors to be used in assembly. The quantities are 
> large enough that there should be no significant cost impact, except for 
> probably a larger overall quantity of spares to be built. Of course, I agree it 
> is simpler to have only one version.
> 	Presently the peaking time of the preamp-cable-ADC125 is about 35 ns I believe. 
> (Maybe a bit more in the case of the cathodes if the detector capacitance 
> affects it; it would make sense but I don't know really.) We might try to reduce 
> it but 14 ns seems to me too small - there will be too much amplitude above the 
> Nyquist zone, this has to degrade timing at some point. Maybe some compromise 
> value like 25 ns would be better to try.
> 	Anyway the starting point for this should probably be to remove all explicit 
> shaping from a channel, hook it up w/ preamp and cable and a test pulser and 
> input loading capacitor, and see the pulse shape. This will exhibit the minimum 
> achievable peaking time; and we can also then we can calculate the shaping time 
> for the ADC board to get to the desired overall peaking time.
> 	Can you describe the algorithm applied to ADC data to get the timing 
> measurement for page 514 work? Is there a fit here, or just level-crossing using 
> the same threshold e.g. 30mV? Certainly an optimal timing algorithm will use 
> more than 2 datapoints from the ADC, i.e., is not simply just a level crossing 
> and linear interpolation between two points.
> 	Is the discriminator simulation 'perfect' or does it include real-world 
> distortions such as time walk (dispersion)?
> 	Sincerely,
>
> 		Gerard
>
>
> Lubomir Pentchev wrote:
>   
>> The minutes of the last FDC meeting were posted at:
>>
>> http://www.jlab.org/Hall-D/software/wiki/index.php/Minutes-9-16-2010
>>
>> Regards,
>>      Lubomir
>>     
> _______________________________________________
> Halld-tracking-hw mailing list
> Halld-tracking-hw at jlab.org
> https://mailman.jlab.org/mailman/listinfo/halld-tracking-hw
>   
-------------- next part --------------
A non-text attachment was scrubbed...
Name: barbosa.vcf
Type: text/x-vcard
Size: 188 bytes
Desc: not available
Url : https://mailman.jlab.org/pipermail/halld-tracking-hw/attachments/20100917/7f5c4993/attachment.vcf