[Halld-tracking-hw] CDC pedestal voltage

[Fernando J. Barbosa]

Hi Naomi,

Every channel will have its own offset due to power supply stability, 
preamp, fADC, etc - this is to be expected. These offsets will also 
drift over time, with temperature, aging, etc. Therefore, there is a 
need to have offset adjustments for every channel in hardware and 
corrections from calibration runs, for example.

The fADC125 already has DACs to set the offsets for each channel 
individually to address these issues and also because of maintaining a 
wide dynamic range. This is already implemented on the fADC125 we have 
here at Jlab for tests.

Best regards,
Fernando





Naomi Jarvis wrote:
> 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.
>   
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