PANDA is one of the main experiments at the future FAIR facility
under construction near the GSI research center at Darmstadt,
Germany. It aims to the study of the antiproton-proton and
antiproton-nucleus annihilation reactions.
The Micro Vertex Detector (MVD) is the innermost part of the
PANDA apparaturs and consists of both silicon pixel and silicon strip
detectors, placed in barrels around the interaction point and in disks
in the forward direction. The two innermost barrel layers and the
central part of the forward disks will be made of Silicon Pixel
Detectors (SPD). Therefore the SPDs will have to cope with the
maximum particle density (up to 6.1 MHz/cm$^2$); the space resolution
requirements therefore limits the pixel size to 100 $\mu m$ x 100 $\mu m$. Moreover,
the PANDA apparatus will not provide a hardware trigger signal, thus
requiring a precise time stamping of the events (1.9 ns r.m.s.) and
a high data bandwidth. Deposited energy measurement is also a requirement
for the SPD readout electronics.
In order to cope with these challenging requirements, an ASIC-based solution
for the readout of the PANDA SPD is under development. The ASIC, named ToPiX,
will consist of a matrix of 116x110 cells, thus covering an active area of
1.32 cm$^2$. The pixel matrix is organized in 55 double columns, each with
independent readout logic and local buffering. An end of column control
logic provides both data transmission via double data rate serializers and
control and configuration informations. The master clock frequency is 160 MHz.
A new reduced scale prototype, named ToPiX v4, has been designed and is currently
under test. The prototype is based on a 640 cells pixel matrix, with a layout
compatible with the previous prototype version. The operating clock frequency
has been increased from 50 to 160 MHz, and a number of modifications have been
made in order to increase the linearity for charges below 2 fC, to linearise the
comparator threshold control DAC and to improve the radiation tolerance with
respect to both Total Ionizing Dose (TID) and Single Event Upset (SEU) effects.
In particular, a combination of DICE cells, Hamming encoding and TMR
redundancy has been implemented in the pixel cell area where space
constraints make the redundancy more critical.
TID and SEU tests will be performed in July-August to assess the effectiveness
of these improvements.
Differential SLVS I/O links have been used to interface the chip. The ToPiX v4
is designed to be compatible with the GBT transceiver under development at CERN
for data and slow control signal transmission.