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Re: Combined tracking meeting



Hi Fernando,  the only think I can see in favor of a 14bit readout is 
that it allows a wide dynamic operational range so that systems with  
widely varying offsets
and gains could potentially be used with the same ADC without requirning 
modification. The resolution I remember for wire chamber measurements
is something like  FWHM = 50% of the measured value.  Probably this is 
gas dependent so a high Z gas like Xenon would give  higher resolution
The only place I might imagine using even 10 bit resolution is in 
position extrapolation between pads. - Mitch

Fernando J. Barbosa wrote:
> Hi Gerard and Mitch,
>
> I think it would definitely be helpful to have some Garfield studies 
> performed on the CDC for the various operating conditions. I am also 
> of the opinion that we should definitely plan for some beam studies 
> with the GAS-2. I know it takes resources but there is a lot involved 
> here. As you know, there is a test beam at BNL that we can plan for - 
> I have used it while at BNL a number of years ago while developing the 
> "predecessor" to the CDC and it provided invaluable data. The next 
> small CDC prototype could be slated for this tests.
>
> I think it is also a good suggestion to ask Mitch about his experience 
> with various developments and if a 14-bit ADC is reasonable or 
> overkill. Mitch could you comment on this?
>
> Certainly the 14-bit option is better than any additional circuitry 
> for a non-linear transfer function. The total cost increase would be 
> about $50k! That's a lot of money, for sure, and this points to the 
> fact that we really need additional studies, as I suggest in the first 
> paragraph. I think you also share that sentiment.
>
> Regards,
> Fernando
>
>
>
>
> Gerard Visser wrote:
>> Hi Elke,
>>     From the quote which I obtained last year, it is about $15 more 
>> per channel to have 14-bit ADC's on the board. There is (according to 
>> spec documents) no change to the power dissipation. [It is apparently 
>> the same chips, just tested to different level of performance that 
>> you buy.] It is probable the price will go down, probably not by 
>> quite that much though.
>>     Nonlinear response will cost much less than $15 per channel, as 
>> long as it fits on the same basic board architecture (which depends 
>> on the details of what we would want to do, of course).
>>     I agree also, the ASIC does some nonlinear transformation, but on 
>> the other hand _that_ may not be very precisely controlled (for 
>> temperature, matching, whatever...). Also, ideally the nonlinearity 
>> would be introduced after all shaping filters so that it can be 
>> precisely and easily un-done by a lookup table in the FPGA, to allow 
>> for integrating the pulse. If the nonlinearity is introduced earlier, 
>> then un-doing it requires some sort of nonlinear digital filter not 
>> just a lookup table.
>>     One final comment, probably it would raise eyebrows to say that 
>> we really need 14-bit resolution from a drift chamber ADC...? We 
>> should for instance get some opinions from Mitch on that. Of course I 
>> will be happy if we do use 14-bits.
>>
>>     - Gerard
>>
>> Elke-Caroline Aschenauer wrote:
>>> On Wed, 23 Jul 2008, Curtis A. Meyer wrote:
>>>
>>> Dear Gerard,
>>>
>>> how much more money are we talking for 14bit vs 12bit. Also keep in 
>>> mind
>>> we will not buy the boards before FY11, so mybe the prise than for 
>>> 14bit
>>> will be the same as now 12bit, but that is speculation on my side.
>>> I also prefer even if there is a bit of more money involved to do 
>>> the easy
>>> straight forward solution instead of a more complicated one. Curtis 
>>> don't
>>> you want for dE/dx all the signal very well measured.
>>>
>>> cheers elke
>>>
>>>
>>>> Date: Wed, 23 Jul 2008 16:49:09 -0400
>>>> From: Curtis A. Meyer <cmeyer@ernest.phys.cmu.edu>
>>>> To: Gerard Visser <gvisser@indiana.edu>
>>>> Cc: GlueX Tracking <tracking@gluex.org>
>>>> Subject: Re: Combined tracking meeting
>>>>
>>>> Hi Gerard -
>>>>
>>>>   thanks for your comments. Actually, the factor is 60, but if we are
>>>> willing to compromise on the slow, forward going protons, we could
>>>> squeeze it.
>>>>
>>>>    I also talked to Fernando about the non-linear aspect, and he also
>>>> mentioned that the ASIC will do that for us at some level. It would
>>>> be good to understand all of the effects involved.  Given the 
>>>> option of
>>>> stuffing the ~45 CDC modules with 14-bit chips might be a good
>>>> option to consider.
>>>>
>>>>   Cheers - Curtis
>>>> On Wed July 23 2008, Gerard Visser wrote:
>>>>> Hi Curtis,
>>>>>     I read your doc, just wanted to mention that as far as the 
>>>>> ADC125 is
>>>>> concerned there would certainly be an option either to have it be 14
>>>>> bits or to apply some nonlinear stage in the analog input signal
>>>>> conditioning. Actually 14 bits has for a long time been planned as 
>>>>> the
>>>>> baseline pcb design so it can be stuffed either for 12 or 14 bits
>>>>> operation, so that all is trivial (except for the higher price of 
>>>>> the 14
>>>>> bit chips). Nonlinear signal processing needs a lot of detailed
>>>>> discussion, it is not going to get into this summer's pcb layout, 
>>>>> but I
>>>>> am pretty sure there will be enough room on the board to add 
>>>>> something
>>>>> if we need to in a future version. Of course if the ADC voltage 
>>>>> scale is
>>>>> made nonlinear the FPGA will also have to transform it back (with 
>>>>> more
>>>>> bits) to linear form before integrating the charge, etc. But this 
>>>>> should
>>>>> be feasible.
>>>>>     Asking for only a factor of 20 in dynamic range sounds low, 
>>>>> but I'll
>>>>> wait to hear details at the meeting...
>>>>>
>>>>>     Gerard
>>>>>
>>>>> Curtis A. Meyer wrote:
>>>>>> Hi Everyone -
>>>>>>
>>>>>>   there will be a combined tracking meeting on Monday July 28 at
>>>>>> 1:30pm. I have created the Wiki page and put a couple of items
>>>>>> on the agenda. Please add appropriate items as people see
>>>>>> fit.
>>>>>>
>>>>>>    - Curtis
>>>>
>>>>
>>>> -- 
>>>> 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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