S. Berglund, C. Bohm, M. Engstrom, S-O. Holmgren, K. Jon-And, J. Klereborn and S. Silverstein
Stockholm University
K. Andersson, J. Pilcher, H. Sanders and F. Tang
University of Chicago

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The purpose of the Tile Calorimeter digitizer system is to digitize PMT signals, obtained from different calorimeter segments. There are 46 PMTs in each barrel module and 32 in the extended barrel. Data acquisition requirements include the following items:
o Analog data is presented to the digitizer system via the 3-in-1 system, which delivers a high and low gain version of each signal, with a gain ratio of 64.
o Data are digitized every 25 ns (40 MHz). Extensive investigations have shown that it is sufficient to digitize the data with 10-bit ADCs, storing only the high gain data unless it overflows or underflows, in which case the low gain data is used. 
o The presence of an interesting event is signaled by a Level-1 accept. This signal is delayed by a fixed first level trigger latency, stipulated to be no longer than 2.5 ms.
o Each triggered event should be recorded over a time frame, with a programmable length of up to 16 samples.
o The choice of gain is the same for the entire time frame, rather than from sample to sample.
o The entire design must be extremely reliable and sufficiently radiation tolerant.

The digitizer system is located in "drawers" inside each TileCal module. The drawers have been designed to contain all the detector-bound electronics. In each drawer the space available for the digitizer is 2.8 meters long and 10 cm wide, with sufficient clearance for two layers of circuit boards. 8 digitizer boards, each serving 6 channels, will fit into the drawer.
The digitizer uses point-to-point connections to connect to the link board. The consequences of a digitizer board malfunction are therefore limited to the possible corruption of its own data. This decoupling of the readout leads to good fault tolerance by eliminating most single point failure modes. 
A simplification in the design is achieved by merging pipeline memories from three channels with a controller into one ASIC, the Tile_DMU, which is implemented in a gate array (from Chip Express). This leads to a considerable reduction of the package count, with the main part of the digital circuitry now contained in three chips on each board: two Tile_DMUs and one TTC-rx chip.
The present version of the digitizer is intended for tests in testbeam during the summer of 1999. After minor modifications the digitizer design should be ready for final production at the end of 1999. 
The 8 boards form two chains of 4 boards read out by a centrally located link interface. The advantages of this scheme include reduced radiation level in the middle of the super drawer and reduced stacking depth. The data is read out via point-to-point links on intermediate boards until they reach the link interface board. Differential LVDS links provide a safe transmission mode with a minimum of digital noise fed back to analog inputs. Surface mount connectors are used to transmit the data.

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The Atlas Tile Calorimeter digitizer is designed to read out 10800 PMT channels. Eight boards (six in the extended barrel) will be placed in each calorimeter module. Each board will receive six dual analog signals (high and low gain) to be digitized by 10-bit ADCs. Two custom designed gate arrays (CX-3061 from Chip Express) contain pipeline memories and derandomizers for three channels each. Selected data words are formatted and sent to Read Out Drivers to be read by the Second Level Trigger. The system is controlled through commands transmitted via the TTC clock distribution system, which sends clocks, trigger information and commands via optical fibers to a TTC-rx chip  on each board, which distributes the clocks and interprets the commands. The system has been designed to achieve good fault and radiation tolerance and a graceful degradation of performance if errors do occur. The design has been developed for test beam tests during the summer of 1999, and is intended for volume production at the end of the year.
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