Pedestal Calculation

# Pedestal Calculation

There are three questions:

## 1) How many events do we need to determine the pedestals and how often do we need to calculate them (i.e. how stable is the SVT)?

Fig 2: Rms and Mean ADC value for Hybrid 1 Wafer 1 from run st_pedestal_1207016.daq

From this study you can see that while the mean ADC value of a given anode changes quite considerably the RMS of the ADC value means to ~1.5 ADC counts showing that the pedestal values are constant. Therefore only a few events are needed to acurately calculate the pedestal

Fig 3: Overlaid plots of the Pedestal calculated from 100 for Wafer 7 Hybrid 1 from Run 1183020_0001.daq and from Run 1183020_0011.daq

The two plots overlaid here are the projection onto the timebucket axis of the pedestals for Wafer 7 Hybrid 1. From this plot we can conclude the pedestals are stable over several hours. Run 1183020 lasted 3 hours and the above pedetals were calculated at the beginning and at the end of this run.

## 2) Whether to subtract the pedestal from the first time bin or whether to subtract the the pedestal with relation to the first capacitor?

Marcelo and Jun are currently studying this question. See their results here .

## 3) Whether to add on an additional pedestal which is a constant for all pixels so we can see the undershoots. We need to do this as the data is written out as unsigned shorts and so we can only save values between 0-255.

- We decided to do this.

Below is a plot showing the average pedestal for all 14 hybrids as a function of days since June 1 2000. Looking at the plot you can see that the average pedestals for all the hybrids is steadily dropping. A look at the actual pedestal per anode for each hybrid (shown via links below) shows that while the value of the pedestal drops the pedetal trends stay constant hybrid to hybrid and anode to anode. It seems the hybrids were "burning in". A study is beeing done to see if this fall in pedestal correlates with the drop in current seen by the slow controls.

Follow this link to see the actual pedestals for wafers 4-7 of the year 1 ladder taken in March 2000

Follow this link to see the actual pedestals for wafers 4-7 of the year 1 ladder taken in June 2000 towards the start of the year 1 running

Follow this link to see the actual pedestals for wafers 1-7 of the year 1 ladder taken in Aug 2000 towards the end of the year 1 running

Helen L. Caines