To build a 256-channel system, all boards, in total 27, are distributed over three separate racks 19 inch wide (see Fig. 2). The main rack, called master rack, contains clock- and bias-driver boards, sequencer, Giga-Link and sync boards. The other two racks, called slave racks, contain sixteen ADC boards, eight boards per rack. Each board accommodates 16 video channels. Because of the bandwidth limitation of the IRACE-PCI card (up to 65 MB/s) it was necessary to split the backplane into two groups. Each group contains four ADC boards and one Giga-Link board, which captures and transmits the data over the fiber optic to the host PCI card. To avoid any timing jitter of ADC convert pulses, the master clock and convert pulses are distributed by the master rack to two slave racks.
2.1 Master Rack
The master rack contains two clock- and bias-drivers, one sequencer, one sync and one Giga-Link module. The Giga-Link module is only used for the command interface to the whole system (i.e. downloading configuration-files, setting voltages, reading telemetry data and status bus controlling). To reduce the number of required boards, several clock- and bias-voltages must share one source. The sync-board distributes the 20 MHz system clock, the convert pulses, the reset signal and four status bus signals to the slave racks.
The slave rack contains eight ADC-boards divided into two groups by splitting the backplane. Each ADC-board accommodates 16 ADCs with a resolution of 16 bits at 500 ksps from Linear Technology. The ADC-board can also be operated in simulation mode, which means that each board can send its channel number or counter value over the backplane. In that way the digital chain can easily monitored, which is essential for the maintenance of a system with such a high number of channels.
To avoid ground loops between the racks, an additional sync-board has been developed. The main component is a digital isolator based on Analog Devices iCoupler® technology (ADuM1401). These isolation components provide outstanding performance characteristics that are superior to alternatives such as optocoupler devices, operating at 100 Mbps. The signals have also been driven by a deferential line driver to be able to use long cables (up to 8 meters).
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