[Halld-tracking-hw] CDC pedestal readout needs

[Naomi Jarvis]

Hi all,

The last email stream on pedestals etc made me a little dizzy.  Here I've tried to get all the CDC's pedestal measurement needs onto one page, to be sure that they are possible with the planned data formats.   I would guess the FDC would have broadly similar needs.   ?


A. Initial setting

Several samples with full range 0-4095 of adc value in order to set the pedestal offsets - I collect a number (10-100) of pedestal samples from 50-100 events and take the mean value.  There are several ways to do this (look at less samples, calculate the mean in the flash etc) but it will be easiest with full range readout available.  Also we need to measure the pedestal width which will be needed in order to set the hit thresholds in the flash.

B. Event data

I need some measure of the pedestal before the hit in order to determine whether it is still high from a previous hit and what is the difference between pedestal and overflow. 

C. Routine pedestal monitoring

Pedestals need to be monitored so that any drifts can be corrected and also to keep an eye on the width (important for hit thresholds).

D. Routine fadc health monitoring

We should take some event data with full range and full precision sample readout to check that the adcs are ok (no bits missing) and check performance of the flash algorithms.  



B.  This is in the pulse data format; almost certainly a subset of the full adc range. 
D.  Would be included in the combined format for pulse data followed by raw sample data as continuation words.  

If we have dedicated runs with a different data format & different trigger & hit-less data then A&C could use the same raw data format.  (We could also make the flash calculate a mean & width for a different pedestal format).

[I thought of monitoring pedestals via the pulse data B or raw data in the combined format for D (if they start before the trigger), but we would only be reading out the channels with hits, so would not see problems if there is a channel without hits because its pedestal has plummeted.]


Can anyone see anything that I have forgotten?

Naomi.