Modifications
This calibration hardware has to modified in order to perform the necessary functions in Run II. The most important modification is the significant reduction of the value of the injection resistor which is located on the preamp board in order to reduce its influence as a parasitic capacitance which becomes larger with the shorter shaping time. Other modifications come from the fact that there will be larger variations in the amount of charge delivered to the preamplifiers from reflections in the pulse distribution system and cable attenuations due to imperfect termination and the skin effect respectively. To prevent any reflection, the calibration signal will be produced as close to the preamps as possible and will travel along a much shorter cable.
It is important to control accurately the signal shape in order to reduce the sensitivity of the calibration to the basic hardware parameters such as the cable and preamp impedance. The calibration current is split into two parts. One goes directly to the preamps and one goes to the electrode and bounces back after twice the cable delay. It is possible to find a calibration pulse which exhibits a similar sensitivity to these parameters.
It will consist of 12 identical subsystems each of which is composed of one pulser motherboard that sends commands and DC currents to two active fanouts. The pulser motherboards control the amplitude and delay of the calibration signal and to select the pulsed channels. Characteristics of the pulser motherboards are that they have one serial bus interface, one channel enable register, one 18 bits Digital-to-Analog Converter (DAC), 6 programmable delays, and 96 DAC controlled current generators. The calibration pulses are generated on the active fanout boards near the preamps. The voltage to current conversion is performed with a low offset opamp, a transistor, and a resistor.
The active fanout is divided into three identical parts. Each part receives one command and 16 DC currents. Each calibration signal is sent via short cable to 4 preamp boards, hitting 6 preamps on each board. One pulser talks simultaneously to 48 preamps.
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