The GSI Event Driven TDC GET4

The GSI Event Driven TDC with four channels GET4 is an application specific integrated circuit (ASIC) designed by the GSI ASIC design group for the CBM Time of Flight experiment. The main aim of this ASIC is to get an high resolution TDC with a time resolution better than 25 ps that can operate in a triggerless data acquisition environment.

The required data rate is about 100 kHz / channel. The double hit resolution should be better than 5 ns and the power consumption less than 30 mW/channel.

GET4 V1

After a first prototype produced in 2008 which was a proof of concept a first full featured prototype is available since summer 2012. The GET4 V1.00 consists of a full custom designed time to digital converter core and a read out system which was synthesised from VHDL code.

The TDC core

The TDC core is base on a delay locked loop structure. This DLL generates 128 phase shifted clock signals with a phase increment of 50 ps. These clock signals are used to sample the four incoming hit signals in hit registers. Temperature code encoders are used to encode the position of leading and trailing edges. These encoders can cope with one hit in each half clock cycle.

The full custom design of this part ensures that the hit signals reach all 128 flipflops of the hit register synchronously as well as that the clock skew of all 128 clock trees is minimised. This results in a minimum of differential non linearities. Complexity of clock and hit signal routing increases with the number of channels. Four channels came out as a maximum for a six metal layer technology at reasonably effort.

Readout

The readout system starts with eight event FIFOs (one for each channel / edge) with a depth of eight entries. The FIFOs are read out with a Token Ring readout. After that two optional readouts are following. There is a 24 bit readout mode compatible to the output data of the very first GET4 prototype and a new 32 bit readout mode that supports all features of the GET4. The 32 bit readout includes an internal time over threshold calculator and slow control readout of internal hit- deadtime and single event upset counter.

Documentation

There is a documentation of the GET4 ASIC available: get4.pdf: GET4 user manual and documentation

-- HolgerFlemming - 17 Dec 2012