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Re: shaper gain



Hi Elke, Elton, Fernando,
	Unless I have really misunderstood, the proposal to reduce the gain of 
shaper board is intended only for tests in the near term, not for the 
final system. That probably will put your mind at ease about the 
nonlinearity and for instance dE/dx capability.
	In the final system, the ASIC "good linear range" at the output should 
be matched to the ADC board input range. (After accounting of course for 
the cable losses and effect of the pulse shaping.) That is, unless we 
decide to have some purposeful, controlled, nonlinear response in the 
system.
	But still, my point is that even for short term tests probably this 
gain reduction by 5x is too much. Of course we can try it, Fernando, but 
if the goal is to see what things would be like with 5x lower gain on 
the ASIC, it might be better to leave the shaper board alone (using the 
"anode" setup of the board) and to put a capacitive charge division 
network between the detector and the ASIC input. That might not be 
practical to do for all channels, for instance there might be crosstalk 
implications, but on just one channel to study the dynamic range it 
would be ok, I think.

	Gerard

Elke-Caroline Aschenauer wrote:
> On Tue, 29 Jul 2008, Elton Smith wrote:
> 
> Dear all,
> 
> I think we have to be a bit careful. If you want to do PID you want to
> have your information as accurate as possible. And a non linear scale
> might impact this, because you compress your information more. This might
> wash out differences you would in a linear scale see.
> I would advise to look into this a bit more before taking a decission.
> 
> bye elke
> 
> 
> 
> 
> 
>> Date: Tue, 29 Jul 2008 08:00:37 -0400 (EDT)
>> From: Elton Smith <elton@jlab.org>
>> To: Curtis A. Meyer <cmeyer@ernest.phys.cmu.edu>
>> Cc: Gerard Visser <gvisser@indiana.edu>,
>>      Fernando J. Barbosa <barbosa@jlab.org>, Elton Smith <elton@jlab.org>,
>>      FDC email list <halld-tracking-hw@jlab.org>
>> Subject: Re: shaper gain
>>
>>
>> Hi Curtis and Gerard,
>>
>> Clearly the ADC and the ASIC outputs should match as best as possible. But
>> my comment is related to the non-linear region, if required, in the
>> data. The main purpose for the dE/dx measurements is to identify protons
>> (distinguish them from pions). For this, we simply need to have pulse
>> height outputs that are different for the two cases, and a non-linear
>> scale may be fine. In other words, non-linear "corrections" per se may not
>> be needed, as we are not trying to determine the precise value of energy
>> loss, simply checking differences between two particle types.
>>
>> Cheers, Elton.
>>
>>
>>
>>
>>
>> Elton Smith
>> Jefferson Lab MS 12H5
>> 12000 Jefferson Ave
>> Suite # 16
>> Newport News, VA 23606
>> elton@jlab.org
>> (757) 269-7625
>> (757) 269-6331 fax
>>
>> On Mon, 28 Jul 2008, Curtis A. Meyer wrote:
>>
>>> Hi Gerard -
>>>
>>>  thanks for the comment on the ADC range. I am not sure what we need to
>>> do, but it would certainly be better to match as much of the linear range
>>> of the ASIC to that of the Flash-ADC.  While we can probably correct for
>>> the lon linear part as we move above the charge limit, I suspect that those
>>> corrections are probably temperature dependent. While this may not be
>>> a problem, we probably want to think about this up front.
>>>
>>>    Curtis
>>> On Mon July 28 2008, Gerard Visser wrote:
>>>> Hi Fernando,
>>>>   Oh, one other point, and this is maybe worthwhile for others to
>>>> consider too. The present "anode" setup of the shaper board, I believe,
>>>> puts the full scale of the ADC at roughly 770 fC (instantaneous input
>>>> charge, e.g. Fe55), as documented on page 2 of
>>>> http://argus.phys.uregina.ca/gluex/DocDB/0007/000736/003/Receiver_Shaper_Setup_for_GPC01.pdf
>>>>   In other words, the "good linear range" of the ASIC is presently about
>>>> 39 to 52% of the ADC range, depending on what you call "good".
>>>>   With the proposed change, the "good linear range" of the ASIC should
>>>> then be about 8 to 10 % of the ADC range. I'm not sure this is really
>>>> what we want to do... (Although of course we can try it and see. I could
>>>> be proved wrong!)
>>>>
>>>>   Gerard
>>>>
>>>> Gerard Visser wrote:
>>>>> Hi Fernando,
>>>>>         Ok, just to check, this will be setting the gain 1/5 of the present
>>>>> "anode" shaper gain, is that correct? (If not, please say...)
>>>> ...
>>>>
>>>
>>>
>>> --
>>> Professor Curtis A. Meyer        Department of Physics
>>> Phone:  (412) 268-2745          Carnegie Mellon University
>>> Fax:    (412) 681-0648            Pittsburgh PA 15213-3890
>>> cmeyer@ernest.phys.cmu.edu  http://www.curtismeyer.com/
>>>
>>
> 
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