[Halld-tracking-hw] [GlueX] Hall D cables

[Gerard Visser]

Hi Yves,
	The ground reference is provided elsewhere (both the preamp/detector and 
separately the readout crates connect to the clean ground). Running it through 
the cables would be a ground loop, and also if any ground currents are present 
(which there always are to some degree) they would then flow in the ground 
connection between preamp board and CDC, which would inject noise by means of 
the CDC capacitance. The ADC has adequate common-mode rejection to reject a volt 
or two of ground noise between the crate and detector, and the high common-mode 
impedance on the ADC input will guard against any unwanted ground currents in 
the preamp board.
	And so the 25th pair is available for the pulser, to which it is connected.

	Gerard

Yves Van Haarlem wrote:
> Hi Fernando,
> 
>>> - What about cables for pre-amp test signals? As the LV cabling still has 
>>> to come and LV + test signals end in a single connector on the HV board, we 
>>> assume test signals will be done together with the LV.

>> The Amphenol cable has 25 pairs and one pair is used for the test signal 
>> which is generated on the fADC125.

> I am confused: the signal coming from the preamp is differential. so I 
> thought you need 24 pairs for the signals + at least 1 single lead 
> (probably one pair) for ground, so there is no room for the test signal? 
> Or what is the grounding scheme?
> 
> Cheers,
>  	Yves