[Halld-tracking-hw] ADC125 as a TDC...

[Fernando J. Barbosa]

Hi Beni,

I see a difference of about 6ns between the timing of the two cards. 
This corresponds to about 3 ft of cable. The cable connectors can only 
be installed on the flat sections of the cable which occur every meter. 
So, it is entirely possible that the cables differ in their lengths by 1 
meter, though they were not supposed to (these are the early prototype 
cables that were assembled on site and there was a limited length of 
cable available). This offset is, however, easily calibrated out. Again, 
this was not the intention here but one can expect delay variations due 
to all the readout components and the detector itself and it is 
opportune to develop a protocol to align all the channels, i.e. calibration.

 From your plot,  the time walk is about 15 ns (by eye) which agrees 
with the peaking time of the preamp (11ns to 12 ns) and the cable 
dispersion for a rise time at the input of the fADC125 of 17 ns. Of 
course, there is an additional effect from the fADC125 shaper. So, I 
think this is to be expected and a correction factor can be developed.

Best regards,
Fernando




Beni Zihlmann wrote:
> Hi Gerard,
> attached is a 2d plot showing for strip 11 the Maximum Amplitude after 
> threshold
> vs the time difference with the wire time (any wire). The threshold is 
> 350 above
> pedestal. (The Max amplitude is pedestal subtracted.) So you see there 
> is a huge
> time walk.
>
> cheers,
> Beni
>> Hi Lubomir,
>>     I think you checked it before when this was first discovered, but 
>> let me ask
>> again because I don't recall it really: If you swap cables at the ADC 
>> board, the
>> timing basically follows the cable? So it is not that the ADC board 
>> has a large
>> time offset between channels? That would be highly unexpected but I 
>> guess not
>> impossible, so it would be good to make this check.
>>     Similarly swapping cables at the preamp board would rule out the 
>> cable as the
>> source of the delay discrepancy...?
>>     If the above is all true, have you tried more than just the two 
>> preamp boards?
>> Maybe one of them is simply broken?
>>     Sincerely,
>>
>>         Gerard
>>
>> On 5/17/2011 12:43 PM, Lubomir Pentchev wrote:
>>> Beni,
>>>
>>> You can also look at the time difference between the top and bottom
>>> strips, compare it with what you have, "t_wire - t_strip", and then you
>>> can extract the wire resolution alone.
>>>
>>> I also don't understand the reason for the big time shift between 
>>> the two
>>> preamp cards, and for the time variations within the cards.
>>>
>>> Lubomir
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>
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