[Halld-tracking-hw] Tracking Meting Tomorrow

[Gerard Visser]

Hi Fernando,
	This is certainly a good idea for testing. I would expect to see reflections at 
the ~+/-5% amplitude level after a round trip cable delay after any signal 
pulse... That should be completely ok for cosmic ray tests, but I'm not sure it 
is what you would want to do for real in a high rate situation. The 
double-termination does serve a useful purpose!
	Certainly we can try it and evaluate the magnitude of the reflections and then 
decide if it is something that we can live with.
	Perhaps a more 'robust' way to accomplish the same goal would be to put a 
series resistors at the ASIC output on the FDC preamp card (the boards are not 
yet in production, right?) and set the ADC125 gain as may be decided for the 
CDC. Then on the FDC preamp only, the resistors are used for attenuation of the 
signals. In this way the double termination can be maintained, reflections will 
be (5%)**2 i.e. about zero. The downside is of course that the ADC125 gain will 
be higher so more sensitive to any external pickup on the cables. But this is 
hopefully not going to be an issue at all.
	Sincerely,

		Gerard

On 4/13/2011 7:47 PM, Curtis A. Meyer wrote:
> Hi Fernando -
>
> please do send us one of these preamp cards -- thanks .. curtis
> On 4/13/11 5:22 PM, Fernando J. Barbosa wrote:
>> Hi Gerard,
>>
>> OK, I will send you two preamps (CDC and FDC).
>>
>> There is an easy way to double the pulse amplitude at the input of the ADC
>> which is to remove the back-terminating resistors on the preamp card. This
>> would not change the preamp response . This change would not require any
>> changes to the fADC125 and one version would still fit both detectors as this
>> is already the consensus at JLab. I can send a preamp to Naomi in this
>> configuration for tests.
>>
>> Best regards,
>> Fernando