[Halld-tracking-hw] Preamp Gain Settings

[Fernando J. Barbosa]

Hi Naomi and Yves,

I just looked at the new pics on the wiki. I got the following peak 
amplitudes from the scope pics (just an approximation) by looking at the 
longer acquisitions:

1850V - Old=800mV, New=350mV, ratio=2.3
1900V - Old=1000mV, New=450mV, ratio=2.2
1950V - Old=1200mV, New=500mV, ratio=2.4.

This shows that the gains are as anticipated. Now, dividing these values 
by the respective gains, the input charge is well over 400 fC in both 
cases and outside of the linear region (for the peaks) but the preamp 
will behave well even if you double the input charge.

The amplitude spectra at 1900V show the peaks at 3500 channels (old) and 
1500 channels (new), again by eye, for a ratio of 2.3. This agrees with 
the above. The spectrum with the new preamp is at about 75% of full 
scale from the saturating point of the preamp (I think it was 2200 
channels previously?).

The energy spectra at 1900V show the peaks at 16000 channels (old) and 
8000 channels (new), eye again, for a ratio of 2. This should not 
compare 1:1 to amplitude spectra but it is approximately what to expect.

Now a note on the operation of the preamp. The impulse response has 
about 11ns peaking time and about 20ns decay (Fe-55,say). For electrons 
streaming into the input, you will get the convolution of this same 
response for every electron. So, you will see a large,short (11ns/20ns) 
pulse if Qin occurs in, say, 1ns; you will get a small,long pulse if Qin 
occurs over a long time window. Therefore, you will need to sum 
(integrate) the samples in the fADC to resolve the total energy deposited.

Thanks for your efforts and best regards,
Fernando



Fernando J. Barbosa wrote:
> Hi Naomi,
>
> Sorry for the delay but I had to look for some old files (old preamp).
>
> The gain setting resistors are correct for both the old (see 
> GlueX-doc-1065 for performance) and the new preamps (GlueX-doc-1364). 
> This means that the gains are: old=1.5mV/fC; new (preamp 4)= 0.57mV/fC.
>
> Best regards,
> Fernando
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