Jens Janssen Diploma Thesis - Prof. Dr. Norbert Wermes ...

Occupancy mod 0 bin 0 - chip 0 col 9 row 8110080604020ScurveFitthreshold 3171.26noise 177.35plateau 100.00VCAL par 0 0.00e+000VCAL par 1 37.78VCAL par 2 0.00e+000VCAL par 3 0.00e+00000 20 40 60 80 100 120 140 160 180 200 220VCALFigure 5.5: The S-curve of a single pixel. Increasing the VCAL DAC lead to adecreased threshold and thus higher hit probability.The standard Threshold Tuning works as follow: a specified amount ofcharge, corresponding to the VCAL DAC setting, is injected into the preamplifier(as described in section 5.4). The charge injection is repeated 100 times foreach scan step and each pixel to gain high statistics. The hit information (hitcounter, which is increased by one if the discriminator asserts the hit signal) isstored in the SRAM. The next scan step, the VCAL DAC value is increased byone, which decreases the threshold, and the process is repeated. By plotting thehit counter versus the VCAL DAC value an S-curve (see figure 5.5) is obtainedfor reasons described in the next paragraph.The Threshold is defined as the number of collected electrons that are neededto make the signal of the preamplifier high enough, so that the discriminatorasserts the hit signal. Ideally, a Heaviside step function can be used to describethis behavior:P hit (Q inj ) = hit(Q inj ≠ Q thr )I(5.6a)=0 Q inj