1. Introduction to the pb Experiment - 7.3.2 PEDESTAL SUBTRACTION

7.3.2 PEDESTAL SUBTRACTION

The determination of pedestal and noise values for each bin of the DSCs are obtained from a "pedestal run", where no signals are connected to the inputs of the DSCs and 1000 events are acquired. The mean and rms values for each bin of the 6 DSCs are calculated according to eq. (6.11), which correspond to the pedestal and noise values.

Figure 7.7: Pedestals p_k calculated as mean values of 1000 events with 0V input. The error bars are the corresponding noise values n_k (rms) of the bins. The variation of the pedestal values is influenced by the GATE pulse, a TTL pulse with a very short risetime. The transients of this pulse are picked up on the input lines of the DSM100. In fig 7.7 the pedestals p_k and noise n_k of bin k for DSC4 are displayed. The resulting values are:

Noise = ±2.0 mV (rms), ( 7.2)

Pedestal variation = ±10 mV (max) . (7.3)

The input of the DSC ranges from -0.8V to 0.8V. The dynamic range is commonly defined as the maximum recordable signal divided by the baseline noise, which determines the smallest detectable signal. The noise value was measured to be 2 mV rms (7.2). The dynamic range of the device is therefore 800/1 which corresponds to 58 dB.

In the analysis routine, the pedestal values for each bin of the DSCs are read from a file and subtracted from the raw ADC-values of each event. In order to reduce the influence of the noise on the calculation of the waveform parameters, the data are smoothed with the following algorithm

, (7.4)

where a_k is the ADC-value of bin k and a_k-1, a_k+1 are the ADC-values of the neighboring bins. This corresponds to a convolution of the data with a discrete gaussian distribution of 2 bins FWHM.