Printable version ATLAS-TRT Front-End Electronics
at Lund University

Barrel boards
v Overview
o Mechanics
v Electronics
o Chips
v Boards
v Barrel
o End-Cap
o Tension-plate
o Stamp-card
o Roof Board
o Chip Development
o Prototype boards
o Software
o Glossary and Index
 
 
o Links

Barrel Front-End Boards

In order to distribute the signals from the straws to the readout electronics, all wires are fixed into brass-fittings which are soldered into through-plated drill-holes in a printed-circuit board. This board, called the tension-plate, has the triple purpose of acting as mechanical support for the readout electronics front-end boards, distributing the straw signals to the ASDBLR and to act as a fixing-plate for the straw-wires. The tension-plate sits directly on the carbon-fibre frame, above the high-voltage kapton board and has the brass-fittings directly placed above the straw positions. The readout electronics daughter-boards are placed into sockets in the tension-plate and the signals from the straws to the inputs of the ASDBLR are routed in copper-traces. The sockets for the ASDBLR-boards are high-quality machined IC-socket strips which are placed without carrier-plastic directly into the tension-plate. The socket pitch is 2mm in order to be able to get enough pins in one row for the ASDBLR-board (22 needed). Since the straw-signals are very low in amplitude and sensitive to noise, several measures have been taken to ensure maximum signal integrity. All traces should be as short and separated as far apart as possible to minimise cross-talk and a ground-plane is placed on top of the tension-plate with signal-traces on the bottom side.

Front-end segmentation

Because of the modularisation of the carbon-fibre frame into three different concentric radial layers, the tension-plate accordingly has to be made as three different shapes. Since the tension-plate also has to cover both sides of the barrel, every shape has to have a mirror-image board which makes a total of sixdifferent tension-plates that has to be manufactured. The designs are called: Inner Tension-Plate, Middle Tension-Plate and Outer Tension-Plate.

The readout electronics that's plugged into the tension-plates are built like a sandwich of piggy- backed boards. Because of the carbon-fibre frame's need for support, each tension-plate is sub-divided into two smaller triangular areas where the daughter-boards can be mounted. On top of each of these areas is a roof-board which distributes power and signals to and from the daughter-boards.

The number of daughter-board sandwiches supported by the roof-boards are as follows:

o  Inner Tension-plate: 21 (10+11).
o  Middle Tension-plate: 33 (15+18).
o  Outer Tension-plate: 50 (25+25).

The straw signals first enter the tension-plate and go through protection resistors and then onto the ASDBLR daughter-board. After the signal is amplified, shaped and discriminated it is fed in the form of ternary differential current signals from the ASDBLR-chip to the DTMROC.

The Roof-boards distribute the signals from the DTMROC's to the ROD's and the slow-control and readout control signals from the ROD's to DTMROCs/ASDBLRs.

The output of the DTMROC uses very low-voltage differential signal levels, and the roof-board converts these into standard LVDS signals. It also converts the incoming control-signals LVDS levels to lower levels for the DTMROC. The roof-board further distributes common signals, e.g. clock, trigger and power by functioning as a buffer/splitter and supplies all DTMROC/ASDBLR boards with these signals.

ASDBLR/DTMROC solution

The readout electronics consists of two different printed-circuit boards, the ASDBLR board and the DTMROC board (Figures 2 and 3). Each board receives the signals from sixteen straws. Each ASDBLR has eight inputs, so two chips are placed on the ASDBLR board and one chip on the DTMROC board. Since the straws are connected to high voltage and might cause sparks, all ASDBLR-inputs must be equipped with a protection-network consisting of a fast diode and a current-limiting resistor. The diodes are packaged in small surface-mount four-diode arrays which are mounted on the front-end board itself, but the protection resistor is too large to be incorporated into the board and must therefore be placed on the tension-plate. The smallest-size resistor is 2.0 x 1.25 mm; smaller resistors are available but would not be able to withstand any accidental high voltage across them. The resistors are placed between every wire connection and the corresponding ASDBLR input socket on the tension-plate. Care must be taken not to block any mechanical structure and to minimise cross-talk. The obvious disadvantage with the resistors on the tension-plate is that since they are placed on the bottom side of the printed-circuit board, it is almost impossible to replace them if they should break. However, this is quite unlikely since the voltage across them and thus the current flowing through them are normally very low.

In order to be able to use the same readout boards on all tension-plates, the physical size and the pin-out of the readout boards was optimised to be 16.5x19mm with 9 pins along each long-side and 2 along the short-side, making a total of 22 pins. Because of their size, the ASDBLR-card and the DTMROC-card are commonly referred to as stamp-cards. This means that the readout chips cannot be packaged but have to be naked chips: the readout boards are made in chip-on-board technology (COB), also known as multichip modules on laminate (MCM-L).

Because of the sensitivity of the ASDBLR-inputs, the digital readout chip was placed on a different COB and the two boards are mounted one on top of the other in a piggy-back fashion. The outputs from the ASDBLR are in the form of ternary differential current and are routed through a connector to the DTMROC-inputs. The connector which combines the 2 boards has to be as small as possible and still have room for the 44 needed pins. Two Samtec FTM/CLM-111 connector with 1 mm pitch are used.

Single ASTRAL solution

The next logical step to simplify design and increase speed as well as reducing cost is a readout solution with only one daughter-board with a chip that combines the functionality of the ASDBLR-chip and the DTMROC, an ASDDTM-chip. The modularity of this chip will preliminarily be 8 channels, so 2 chips on each daughter-board will be necessary in order to maintain compatibility with the current design. Since this is the most probable solution in the next version, all connections on the current piggy-back version have been chosen to work for both types of boards. If a 16-channel ASTRAL chip is available, the board lay-out will be simpler.

This type of board will not be produced until the ASDBLR/DTMROC-version has been fully debugged and tested in the beam.



The current version of the Barrel front-end electronics consists of four different modules:
Inner Tension-plate, signal side A Tension-plate connecting to the actual barrel-chamber straw and distributing the signals.
ASDBLR daughter board Daughter-board containing 2 ASDBLR-chips and receiving 16 straws.
DTMROC daughter board Daughter-board containing 1 DTMROC-chip which receives the signals from the ASDBLR and distributes them to the Roof.
Roof-board Roof-board, which collects the signals from the DTMROCs and distributes power, control and read-out signals from Local Logic.
All of these boards are mounted together in a sandwich construction with straws in on one end and Local Logic connectors on the other.

ATLAS
©1997 Particle Physics Department, Lund University
Comments to: Lund Electronics group, bjorn@quark.lu.se