A Readout System Utilizing the APV25 ASIC for the Forward GEM Tracker in STAR

1. A Readout System Utilizing the APV25 ASIC for the Forward GEM Tracker in STAR G. J. Visser1, J. T. Anderson2, B. Buck3, A. S. Kreps2, T. Ljubicic4 1Indiana University Center for Exploration of Energy and Matter, USA 2Argonne National Laboratory, USA 3Massachusetts Instituteof Technology, USA 4Brookhaven National Laboratory, USA http://www.star.bnl.gov/ Introduction The Forward GEM Tracker (partially installed and commissioning now in STAR Run 12) will extend STAR’s tracking reach to η=2 (covering the EndcapEMC), enabling charge sign discrimination for e+/e−from W+/− decay. When studied with longitudinally polarized proton beams, Wproduction parity violating asymmetries are an ideal tool to study the spin-flavor structure of the proton and hence an important part of the spin physics program at STAR. The FGT, located inside the inner field cage of the STAR Time Projection Chamber (TPC), is a set of six independent triple-GEM detector disks, consisting of four independent quadrants pinned to a common support. For each quadrant, contained in its own working gas pressure volume with independent bias voltage divider board, a planar strip / pad (two dimensional readout) padplane fabricated on kapton film with two copper layers is used, in conjunction with the triple GEM structure, to detect the ionization signal of charged particle tracks and provide high spatial resolution (<100 µm) readout. The outer (inner) radius of the active area of an FGT quadrant is 381 (115) mm, while the perimeter frame is only 13 mm wide. The “Phi-projection” of charge is sensed with 720 strips on the padplane, while the “R-projection” of charge is sensed with 560 “lines” of pads interconnected with lines on the routing layer of the padplane, for a total of 1280 channels per quadrant. The front-end electronics assemblies, described in detail in section II, Fig. 2. The FGT front-end board. The layout of the readout interface signals is symmetric to the two ends of the board, since it is used in both orientations on the FGT detector. The 640 input signal route to 640 card-edge connector pads along the bottom edge. Ground is connected from the detector to the two pressfit nuts soldered to pads on either side of the middle APV chip. effectively “straddle” the gap between quadrants, with four front-end assemblies per disk, and a common ground for the whole disk. This ground is locally isolated from the other disks and the support structures; each of the disks is grounded through its front-end cables to the substantial clean gr Noise and nonlinearity Open input: Noise and linearity are evaluated with open input and with 14.9 MHz sine input. Features σ = 0.52 σ = 0.43 • 72 channels in a 6U VME64x/VXS module, $55 per channel • Assembly option for 12 or 14 bits • Up to 125 MSPS, internal or external sample clock • Differential input 440 mV full scale • Cable loss equalization • Shaper/anti-alias filter 24 ns peaking • Independent offset DAC per channel • Preamp test pulser output • Acquisition buffer: Dual-port circular buffer, 2048 points (19.5 μs) • FPGA-based signal processing • 1 MB buffer and 2eSST readout • Remote firmware upload (VME) Single event 1000 pt readout cable to FEE σ = 5.62 (12-bit: 1.40) 11.5 ENOB Nonlinearity ≈ 0.05 % Receiver / freq eq. Crosstalk System response Preamp + 18 m cable + ADC Shaper 15 MHz* sine input, single event 100 pt readout DAC per channel ADC *Corresponds to peaking time ≈33 ns for combined system (preamp, cable, ADC) THERE ARE MANY ELEMENTS OF OLD POSTER HERE STILL TO BE CHANGED/DELETED!! • Design choices • Some compromises are necessary for maximum channel density. The analog signal path must be kept relatively simple. • In particular: • A passive shaper uses no power, little board area • An ADC driver stage is not necessary if we can keep the drive impedance reasonably low without it • Differential drive is not necessary if the ADC uses a (relatively) high VDD and if we can tolerate a little nonlinearity • However we do not compromise on the line receiver, this is the most crucial element. It is important to: • Receive differential signal accurately • Not inject common-mode ground noise back to cable & front end electronics • Equalize cable frequency response 21 hostile (red) and 1 hostile (blue & brown) Indicates ~2% crosstalk in neighbor channels, acceptable but will try to reduce with layout improvements Mezzanine board Main board ADC FE FIFO ADC FE FIFO ADC FE FIFO • Current status • 3 modules assembled & tested • Interim firmware (FE deadtime, no block transfers) • Deployed in support of CDC and FDC development • Next steps: • Full deadtimeless readout, 2eSST transfers • Minor changed to layout (obsolence, crosstalk) • Develop production test • Pre-production run (20 modules) in early 2011 • 188 modules (+spares) in 2012 FE (Front-end processing): This includes acquisition buffer writing, point extraction, lookahead zero suppression, channel number & header insertion ADC FE FIFO … (36 ch total) … … (36 ch total) … Channel data FIFO: 1024 words (16 bits + 2 flags, internal use only) ADC FE FIFO ADC FE FIFO ADC FE FIFO ADC FE FIFO ADC FE FIFO ADC FE FIFO Each 160 MB/s i.e. 400 words/evt @ 200 kHz 320 MB/s 1 MB Frontend FPGA XC3S500E-4 12 total, 6 ch each Processor FPGA Output FIFO VME slave 2eSST etc. frontend FPGA FEE Power Supply ±1.8V 2.5A remote reg. XC3SD3400A-4 160 MB/s main FPGA ADC (quad) The bottom line… to readout / control backplane (CPCI style) 0 – 360° clock phase shifter (4×) for APV & ADC opto-isolated I2C master (2×) cable to FEE analog line receiver / filter Readout module for 2×12 APV (3072 det. channels) The PCB is 10 layers w/ 6 planes (3 GND), 0.003” buried capacitance. Typical cluster in R projection (timebin on X axis) Complete integrated readout and power system for FGT This work was supported in part by the Offices of NP and HEP within the U.S. DOE Office of Science and by the U.S. NSF. †GAS II: A Versatile Wire Chamber Readout ASIC. N. Dressnandt, N. Doshi, M. Newcomer., IEEE NSS 2009.