  For the upgrade of the inner tracker of the BESIII spectrometer, planned for
2018, a lightweight tracker based on an innovative Cylindrical Gas Electron
Multiplier (CGEM) detector is now under development. The analogue readout of
the CGEM enables the use of a charge centroid algorithm to improve the spatial
resolution to better than 130 um while loosening the pitch strip to 650 um,
which allows to reduce the total number of channels to about 10 000. The
channels are readout by 160 dedicated integrated 64-channel front-end ASICs,
providing a time and charge measurement and featuring a fully-digital output.
The energy measurement is extracted either from the time-over-threshold (ToT)
or the 10-bit digitisation of the peak amplitude of the signal. The time of the
event is generated by quad-buffered low-power TDCs, allowing for rates in
excess of 60 kHz per channel. The TDCs are based on analogue interpolation
techniques and produce a time stamp (or two, if working in ToT mode) of the
event with a time resolution better than 50 ps. The front-end noise, based on a
CSA and CR-RC2 shapers, dominate the channel intrinsic time jitter, which is
less than 5 ns r.m.s.. The time information of the hit can be used to
reconstruct the track path, operating the detector as a small TPC and hence
improving the position resolution when the distribution of the cloud, due to
large incident angle or magnetic field, is very broad. Event data is collected
by an off-detector motherboard, where each GEM-ROC readout card handles 4 ASIC
carrier PCBs (512 channels). Configuration upload and data readout between the
off-detector electronics and the VME-based data collector cards are managed by
bi-directional fibre optical links.
