Re: fadc dynamic range of FCAL

["Fernando J. Barbosa" <barbosa@jlab.org>]

Hi Mark, Beni and Elton,

It was decided at the last collaboration meeting to require the ADCs as 
12-bit.

However, a discussion on the number of bits (resolution & dynamic 
range)  is  not particularly interesting without considering the 
practical issues. If the peak amplitude out of the PMT were to be 5V, 
the LSB would be 1.22 mV. This implies that the lowest 10 bits, at 
least, would be useless.

Beni, what are the pulse characteristics you observe with the present 
setup? - (Noise, peak, average, rise time, decay time constant) And are 
these similar to what you would expect in the final implementation?

Regards,
Fernando

Elton Smith wrote:
> HI Mark and Beni,
>
> We have to be careful. If the flash is 10 bit, then if we set the max
> (1024) to correspond to 9 GeV (sum measured with effective 12 bits), then
> 1 bit corresponds to 8.8 MeV. Therefore, without saturating the maximum
> pulses, the resolution on the lowest values suffers. It is true, however,
> that 30 MeV would have about 4 samples with values of about 1 bit each.
> Not too good.
>
> 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 Sun, 3 Feb 2008, Mark M. Ito wrote:
>
>   
>> If you have a 10-bit flash, and your pulses show up in 4 time slices,
>> then you have roughly a 12-bit dynamic range. That is a factor of 4096.
>> If you set the top at 9 GeV then the least count is 2 MeV. So you
>> measure your 30 MeV in about 4-bits, which is not bad.
>>
>> 9 GeV is probably more than you will ever see, so that spec can probably
>> be relaxed. Plus there is the trick Elliott mentions for the rare whopper.
>>
>> Note also that we are not talking about firing a discriminator with a
>> threshold. The FADC info will be there, it is just a matter of how quiet
>> the pedestal is.
>>
>>
>>     
>
>   
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