Re: FDC/CDC ASIC meeting minutes

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

Hi Yves,

This is great news. Your measurements are indeed close to what we 
measured with the FDC. The problem with the method you were using is 
that the average gets biased by saturating pulses and system noise. 
Saturating pulses at the preamp are also completely out of the linear 
region and will skew the results, especially with regards to the chamber 
gain. I also need to verify if there is a change in shape as the preamp 
goes into the non-linear region.

I will have the interposer with the gain decreased by half for you next 
week.

Regards,
Fernando



Yves Van Haarlem wrote:
> Hi Fernando,
>
> I did the measurement we discussed: measuring the shaper-gain with 
> signals within the linear range of the preAmp. I used cosmics (because 
> we know that the Fe-55 X-rays will saturate the preAmp) and selected 
> signals which are in the linear range of the pre-amp (<450 mV 
> amplitude), for different HV settings. I did 3 measurements for every 
> HV setting - I also took care that the signals had kinda the same 
> shape (no noise) and that there were no double pulses that were close 
> to each other. I saved all traces in case somebody wants to look at 
> them. I'll list the average shaper gains (out of three measurements), 
> I measured:
>
> HV(V) | shaper-gain
> ==================
> 1600  | 2.3
> 1650  | 2.4 1700  | 2.3
> 1750  | 2.5
> 1800  | 2.4
> 1850  | 2.1
> 1900  | 2.1
> 1950  | 2.1
>
> ==> the error on the shaper-gain is around 0.1 (sigma/sqrt(3)), I 
> first measured the amplitudes at the different HV settings and 
> afterwards calculated the shaper-gains to keep myself as unbiased as 
> possible.
>
> I believe the shaper gains are fairly constant and indeed the shaper 
> has the same gain for equally shaped signals. As I understood this is 
> the same measurement the FDC did. I am wondering if they would see the 
> same shaper gain behavior as a function of HV if they took an average 
> over a lot of signals for different HV settings (using the Fe-55 
> source) as I did before this measurement, as that gain should be 
> constant if the Ar peak is in the linear range of the preAMP for all 
> FDC HV settings.
>
> If I have the interposer with the charge division I could repeat the 
> measurement with Fe-55 source (if it is in the linear range of the 
> preAmp).
>
>
> Cheers and have a nice weekend,
>     Yves
>
>
>
>                           /--------------------------------
>                          /   Yves Van Haarlem
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