Speaker
Description
Summary
Particle identification in nuclear physics experiments requires the digitisation of analogue data generated by the detectors, such as time, amplitude and charge information. Up to now digitisation of data is done mainly by Application Specific Integrated Circuits (ASICs), which requires a long time and a lot of resources for development. With the recent developments in the Field Programmable Gate Array (FPGA) technology, it is now possible to use FPGAs for data digitisation by using complex commercial electronic components for the necessary Front-End Electronics (FEE). We call this concept come & kiss: COMplex COMmercial Elements & Keep It Small and Simple.
To gain the time information (e.g., Time-of-Flight (ToF) or Time-over-Threshold (ToT)) of a signal generated by the necessary detectors, it is pre-amplified using available cell phone amplifiers to a certain level. The amplified signal is digitised in an FPGA differential input buffer used as a discriminator. By this configuration the time information of the particle is encoded in the rising and falling edges of a digital pulse. ToF measurements can be carried out by measuring the rising edges of 2 signals, whereas the ToT measurements can be done by measuring the pulse width of the discriminated signal. For precise time measurements a Time-to-Digital Converter (TDC) is implemented in the FPGA with high time resolution (<15 ps RMS).
For amplitude measurements an Analogue-to-Digital Converter (ADC) can be implemented in a FPGA by means of a TDC. The signal to be measured is applied to a differential input buffer of an FPGA with a reference voltage generated by a linear sawtooth generator on the second input. The differential buffer behaves as a comparator and generates a digital signal at the crossing of the reference sawtooth signal and the analogue signal. From the shape of the reference signal and the switching time measured by the TDC the amplitude of the input signal can be calculated.
Charge measurements are done in a similar fashion. The charge of the signal above the threshold is encoded in the width of a digital pulse via a constant current discharge of the integrated signal. The width of the pulse is proportional to the charge of the analogue signal. Using a TDC the width of the pulse is measured and the charge value is calculated.
The FPGA based data digitisation has advantages over the traditional ASIC solutions as well as disadvantages. The components used for a come & kiss system, basic cell phone amplifiers and FPGAs, are cheap and are always easily available removing the risks of mature devices. This method doesn't require as much amount of time, resource and cost as an ASIC design and has the advantage of being easily adaptable to different requirements. Of course, creating a solution by using commercial components is not as small as a special ASIC solution.
For all described digitisation methods test boards were developed and the results and limits are presented.
