The comparator

 The comparator (fig. 5) with programmable signal polarity is located behind the frontend to detect hit channels. To supress frontend pedestal fluctuations the comparator has been AC-coupled with a large time constant. The output of four neighbouring channels is ORed, latched on the positive edge of CompClk and brought offchip as open drain signals notCompOut$\langle$31...0$\rangle$ which can drive 4mA at maximum .

In case of radiation damage the following strategy is recommended:

• VcompRef controls the comparator threshold level (1 MIP$_{\rm Si}$ $\approx$ 50mV peak height, so a value of 20mV is a good start to detect MIP signals); due to the AC-coupling this level is independent from radiation induced frontend baseline shifts and so it should not be altered.
• Icomp sets the comparator bias current. It can be increased slightly under irradiation (however, no strong effects expected)

  

Figure 5: AC-coupled comparator