An important part of our detector with no access during most of the year is a sophisticated monitoring and calibration system. These control tasks are also performed by the decoder chip.
Calibration signals can be distributed to each preamplifier input of the APC128 by means of a shift register on the decoder chip running synchronously with the Pipeline Shift Register on the APC128, and the 32 bit Calibrate Register which determines which of the 32 pipeline buffers will be filled with the calibration pulse.
The decoder chip is equipped with a DAC that is connected to the seven bit Current Supply Register. The output of the DAC drives a current source on the hybrid (pnp-transistor SA MMBTA56) for the preamplifiers on the APC128 chip.
For calibration of the analog readout chain and other control functions,
the LED driver can be supplied sequentially with a set
of voltages (see figure
3) denoted by Temp.Control, V 
,V 
and V 
in figure
5. The voltages V and V are derived from a reference
voltage obtained from an AD580 chip on the hybrid, and
serve for calibration of the transmission line. V is the
voltage for the APC128 preamplifiers as generated by the current
source. A drift of V at
fixed current can be caused by radiation-induced threshold voltage
shifts in the CMOS transistors. By repeated reloading of
the 8 bit Slow Control Register, V , V , V
and an NTC resistor for
temperature measurement are connected to the output stream, and the data
appended to each event.