Testing of the PASA Chips of the ALICE TPC

The “time projection chamber” (TPC) [1] as one of the main components of the ALICE detector has a cylindrical shape with a diameter of 5 m and a length of 5 m. It is composed of several individual TPC sectors. Altogether 570 000 read-out channels have to be equipped with electronics which requires 4500 “front-end cards” (FEC) each having 8 pre-amplification units (PASA) and 8 ALTRO chips. To improve the reliability of the boards, all PASA and ALTRO chips have been tested before loading the FECs. The completed FECs are then tested before assembling the TPC [2]. For testing the PASA chips, a test setup has been developed by the electronics workshop of our institute. It consists of a main board which is controlled by a PC. This board produces 16 variable signals (one “stimulus” per channel) which are sent to the PASA chip under test. The output signals of the PASA are read and digitized by 12-bit ADCs with 40 MHz simultaneously for all channels on the board. The results are transferred to a PC and a test software which has been written as part of a diploma thesis [3], calculates gain, noise, linearity, cross talk, shaping time, etc. This information allows to sort according to performance. In total close to 50 000 PASA chips were tested. These tests were performed using a robot available at the University of Lund, used before to test the ALTRO chips. The robot was handling the insertion of the chips in the socket of the test board and the subsequent sorting. A crucial point are reliable contacts between the pins of the chips and the socket, achieved by a spring loaded socket. A mechanical device operated by compressed air and controlled by the PC released the socket for changing the chips. The PASA chips have been sorted according to the following criteria: a chips is “bad”, if the current exceeds the limits or if the performance is more than 20% off the expected values. Those chips were sorted out. For the use in the ALICE TPC further criteria have to be fulfilled: