The MDT system consists of about 300,000 high-pressure drift tubes 3cm in diameter, ranging in length from 1.4m to 6.3m. The primary function is muon tracking, with a specified resolution per tube of rms. In addition to leading edge timing, other information such as trailing edge timing, pulse amplitude, and multi-track data may be extracted from the drift tube signal. The baseline readout allows one of these additional pieces of information to be transmitted to the DAQ system, and the choice of which one may be a programmable option. This paper describes the current development status and prototype test results; for more information on the system see.
The MDTs are read out from one end by a charge-sensitive pre-amplifier, and the other end is terminated at the characteristic impedance of the tube (about .) The pre-amplifier is contained within a custom 8-channel ASD IC, which also integrates a timing discriminator, and a simple ADC. The ASD outputs are logic signals which drive the inputs of a multi-channel TDC (time-to-digital converter). Both ASD and TDC chips are mounted on a PC board which is directly connected to the end of the MDT chamber. The high-voltage decoupling capacitor will be mounted in the tube end-plug to simplify the PC board design.
The data from the TDC are transmitted via an 80MB/sec serial link to a VME crate located on the detector. The crate houses a custom read-out driver (ROD) which can accept data from up to 64 TDC chips. The data is re-formatted and transmitted via 1GB fiber-optic link to the ATLAS data acquisition. A pictorial diagram of the readout system is shown in Figure 1.
Several functions of the ASD and TDC may be programmed by the ATLAS DCS (detector control system). All IC's on the front-end PC board will be interconnected using the JTAG boundary scan protocol, which will also be used for production testing of the boards.
Figure 1: MDT Readout System