A 72 Channel 125 MSPS Analog-to-Digital Converter Module for Drift Chamber Readout for the

GlueX DetectorG. Visser1, D. Abbot2, F. Barbosa2, C. Cuevas2, H. Dong2, E. Jastrzembski2, B. Moffit2, B. Raydo2

1Indiana University Center for Exploration of Energy and Matter2Thomas Jefferson National Accelerator Facility

IntroductionThe GlueX detector system, now under construction

for Jefferson Lab Hall D includes the 3098 channel straw tube central drift chamber (CDC) surrounding the target, and the 24 layer planar cathode strip forward drift chamber (FDC) in the forward region. The FDC has 2304 anode wires with discriminator/TDC readout and 10368 cathode strips. An 8 channel frontend ASIC has been developed for these detectors in TSMC 0.25 µm CMOS technology.†

The CDC and the cathode strips of the FDC, a total of 13466 channels, require 12 bit ADC (waveform digitizer) readout capable of deadtimeless operation at up to 200 kHz trigger rate, providing drift time measurements with <2 ns rms error, pulse amplitude measurement for position interpolation from cathode strips in the FDC, and pulse integral measurement for dE/dx determination in both detectors. Hit rates may be up to 300 kHz in some FDC channels. The combination of maximum drift time ~1 µs and trigger rate requires a readout system capable of assigning one pulse to multiple events, an ambiguity to be resolved only later in track reconstruction.

We have developed a high channel density VME64x solution for the drift chamber readout ADC, described here. It supports a sample clock up to 125 MHz.

Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. The U.S. Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce this manuscript for U.S. Government purposes.

Design choicesSome 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

Receiver / freq eq.Shaper

ADC

σ = 5.62 (12-bit: 1.40)11.5 ENOB

Open input:

Typical FDC event (preliminary data from FDC prototype)ADC PS noise/ripple <1mV

1.026 MHz

20 MHz BWL

15 MHz* sine input, single event 100 pt readout

21 hostile (red) and 1 hostile (blue & brown)

Indicates ~2% crosstalk in neighbor channels, acceptable but will try to reduce with layout improvements

A linear regulator for ADC power (3.00 V @ 13 A) is impractical for this module. Solution: a low noise three phase buck converter using integrated synchronous 8A chips and an external reference and error amp. Achieves <1mV output noise and ripple. Switching frequency not detectable in ADC output noise spectrum. Measured efficiency is 89%.

DAC per channel

σ = 0.43 σ = 0.52

Noise and linearity are evaluated with open input and with 14.9 MHz sine input.

Nonlinearity ≈ 0.05 %

ADC FE FIFO

ADC FE FIFO

… (36 ch total) …

ADC FE FIFO

ADC FE FIFO

ADC FE FIFO

ADC FE FIFO

ADC FE FIFO

… (36 ch total) …

ADC FE FIFO

ADC FE FIFO

ADC FE FIFO

Mezzanine board

Main board

Processor FPGA

Output FIFO

VME slave

2eSST etc.

Each 160 MB/s i.e. 400 words/evt @ 200 kHz

160 MB/s

1 MB

320 MB/s

FE (Front-end processing):This includes acquisition buffer writing, point extraction, lookahead zero suppression, channel number & header insertion

Channel data FIFO:1024 words (16 bits + 2 flags, internal use only)

XC3SD3400A-4

Frontend FPGA

XC3S500E-4

12 total, 6 ch each

Note: Output ripple is observed mainly at the per-phase switching frequency. The imbalance ripple dominates the ideal sum ripple! Nevertheless small enough.

Power The bottom line…

Noise and nonlinearityFeatures• 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)

PECL/LVDS CLK & TRG input from VXS or P2

Processing FPGA

Total power dissipation 58 – 68 W per module

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

ADC (dual) Frontend FPGA

Crosstalk

Single event 1000 pt readout

*Corresponds to peaking time ≈33 ns for combined system (preamp, cable, ADC)

Two 10-layer boards w/ 6 planes (3 GND), 0.003” buried capacitance.

†GAS II: A Versatile Wire Chamber Readout ASIC. N. Dressnandt, N. Doshi, M. Newcomer., IEEE NSS 2009.

http://www.gluex.org/

System responsePreamp + 18 m cable + ADC

VME FPGA

±5.5 V supply (for receivers)