  Readout chips of hybrid pixel detectors use a low power amplifier and
threshold discrimination to process charge deposited in semiconductor sensors.
Due to transistor mismatch each pixel circuit needs to be calibrated
individually to achieve response uniformity. Traditionally this is addressed by
programmable threshold trimming in each pixel, but requires robustness against
radiation effects, temperature, and time. In this paper a self-adjusting
threshold mechanism is presented, which corrects the threshold for both spatial
inequality and time variation and maintains a constant response. It exploits
the electrical noise as relative measure for the threshold and automatically
adjust the threshold of each pixel to always achieve a uniform frequency of
noise hits. A digital implementation of the method in the form of an up/down
counter and combinatorial logic filter is presented. The behavior of this
circuit has been simulated to evaluate its performance and compare it to
traditional calibration results. The simulation results show that this
mechanism can perform equally well, but eliminates instability over time and is
immune to single event upsets.
