The charge produced by the passage of ionizing particles through the bulk of the detectors is collected on segmented anodes, with a pitch of 250 m, on the far edges of the detector. The 240 anodes on both ends of the detector are wire-bonded to an electronics carrier which houses bipolar preamplifier/shaper chips (PASA) and a CMOS based switch capacitor array (SCA) used as an analog memory pipeline.
In 300 m of Silicon a minimum ionizing particle traversing at normal incidence will generate 24,000 electrons. However, diffusion and Coulomb repulsion limits the signal in a single pixel to not more than 5,000 electrons. The incoming signals from the SDD are preamplified and shaped in the PASA. The intrinsic noise of the readout chain does not exceed 400 rms electrons, leading to a signal to noise ratio of 60:1. Detector response simulations show that this ratio is required to achieve excellent position resolution. The complete readout chain is designed to have a dynamic range that allows measurement of the energy deposition of minimum ionizing particles (1 MIP) as well as low momentum ( 100 MeV/c) protons ( MIPs).
The shaped signals are stored in a switched capacitor array as a function of arrival time. The SCA is clocked at a frequency of 40 MHz and has a depth of 256 time-buckets. It is operated as a circular buffer to avoid the loss of drift space implied by the 1 s 1st level trigger delay.
Each PASA and SCA chip hosts 16 channels. The chips will share an on detector hybrid mounted on a 2 cm wide Beryllia substrate to reduce radiation length on the detector. The inclusion of the SCA on the detector provides multiplexing capabilities leading to a drastic reduction (factor 240) of the number of cables to be attached to the detector and hence not only simplifies the design but allows a reduction of the effective detector mass.
The SCA output is subsequently transmitted to the readout boards (RDO) located at the TPC end caps. Each readout board houses ADCs and a fiber link driver used to send the 10 bit ADC output to the DAQ. Part of the RDO is a power conditioning sector which distributes all required low voltage power levels (5 V,8 V,-2 V,+3 V, +12 V).
The two functional block diagrams shown in figs. 14 and 15 represent the on- and off-detector components respectively.
Figure: Functional block diagram of on-detector electrical components
Figure: Functional block diagram of off-detector electrical components
High voltage will be supplied to voltage dividers via separate 2.5 kV lines. The dividers are located at the water manifolds on each side of the SVT. Each divider board serves one ladder, which results in 36 lines. The boards are connected via Kapton cables to the first detector on each ladder. The support voltages are supplied on Aluminum traces with a 1.2 mm pitch. Each 10th cathode is externally powered, leading to 50 traces per Kapton cable.
The present SVT design is an integral part of the STAR integration system. Cable routing, coolant supply, power requirements and crate space were taken into account by the STAR integration group in their layout of the complete system.