[Halld-tracking-hw] CDC pedestal voltage

[Naomi Jarvis]

Hello,

I have a question about pedestals for you...

In order to make best use of our dynamic range I need to set the pedestal as low as possible, whilst allowing room for some variation to ensure that it does not fall off the bottom of the ADC range.  There is one adjustment on the shaper which moves the baseline of all 16 channels together, but the channels have widely differing pedestals - 500 channels between the highest and lowest.

In the final ADC+shaper unit, would the offsets be adjustable individually? 

Would we need this?  

I am not sure if the variation is caused by the straws, the preamp or the shaper.  I know the two ADC offsets are around 100 channels different.  I switched preamps (our new one - preamp4 - and another - preamp3 - which no-one has heard much about (!) from Gluex 1364 Table 2 row 1 because we opted for the row 2 preamp because it has lower gain) to see if the variation was mostly coming from the shaper, but this appears not to be the case.  

It would be nice to start with the straws and look at the pedestal voltage directly but I don't think this is possible.  I could look at the preamp output for some channels by sacrificing an old cable which has just 8 channels connected.  I wondered if anyone had any idea what might be causing the pedestals to be so different. ??

I have put some pics online http://www.jlab.org/Hall-D/software/wiki/index.php/Pedestal
The preamp switch plots are at the end of the page, linked to 
http://www.jlab.org/Hall-D/software/wiki/index.php/Pedestal#One_event.2C_different_preamps

Thanks for any suggestions

Naomi.