Speaker
Description
Summary
The TDCPix is a hybrid pixel detector readout ASIC designed for the
NA62 GigaTracker detector. The driving requirements are a single-hit
timing resolution better than 200ps RMS and a hit loss of less than
1% in the presence of a (highly non-uniform) beam rate up to
80MHz/cm$^2$. This hit rate leads to an expected data rate at the output
of the chip which can reach 6Gb/s.
The TDCPix comprises an asynchronously operating pixel array with 1800
pixels organised as 40 columns of 45 pixels, each one 300x300$\mu
m^2$. The front end input stage has been optimised for a detector
capacitance of 250fF and the predicted ENC with the fully depleted
detector is better than 250$e^-$. The base-line detector is expected
to be a standard P-on-N planar pixel detector 200$\mu m$ thick,
although the front end remains programmable such that other detector
technologies may be evaluated. The pre-amplifier was designed with a
peaking time of 5ns to provide the required timing performance. Rather
than distribute a high precision reference signal to the pixel array,
the discriminated hit signals are driven to the end of column region
via dedicated transmission lines. There, each column is instrumented
with a 720 channel Delay Locked Loop based time-to-digital converter
(TDC) with a nominal bin size of 97ps. The TDC measures both the
discriminator leading and trailing edge times permitting a
time-over-threshold approach to the required discriminator time-walk
correction.
Hits from the TDCs are sent to data buffering and concentrating
logic. The read-out uses four 3.2Gb/s serialisers with the high speed
clock being provided by a low-noise on-chip Phase Locked Loop. The
high data rates negate the possibility of buffering the data stream
whilst awaiting a trigger, thus a self triggering architecture with
continuous data readout has been adopted.
All configuration and state logic in the design deemed critical for
the correct operation of the chip has been triplicated to provide
increased single event effect tolerance. On-chip digital-to-analogue
converters provide threshold generation and trimming and are
configurable through a single-signal configuration interface. The
configuration and DAQ interfaces include a DC-balanced protocol layer
permitting direct optical connections when the detector assembly is
installed in the experiment. Dedicated calibration circuitry is
included to enable the required timing resolution to be reached.
The chip was manufactured in a commercial 130nm process during the
second half of 2013 and testing began at the start of 2014. Bump
bonding of the TDCPix chip to a P-on-N detector is currently being
scheduled.
A detailed description of the ASIC architecture and performance will
be presented alongside results from the single chip assembly, if it becomes
available in time.
