A Comparison of Voltage-Mode and Time-Based Timing and Energy Read-Out Circuits

Modern applications like nuclear measurements, and energy discriminating detectors, to name a few of the most prevalent, have all witnessed a significant increase in timing measurements. For this purpose, high-performance Time-to-Digital Converters (TDCs) play an increasingly key role. However, in many setups, the measurement of occurrence time of an event is accompanied by the measurement of the energy associated with it. Therefore, the ideal would be to combine the measurements of the two parameters with the performance required by the specific experimental context. In literature, entirely time-based solutions are available, such as the Time-over-Threshold (ToT) technique, or mixed solutions in which the TDC, used for timing only, is coupled with a voltage-mode analog circuit for energy readout (a.k.a., amplitude of the signal), such as a PeakDetector (PD) or an integrator (INT) that interfaces via Analog-to-Digital Converter (ADC) to a digital system. This paper investigates the performance obtained in terms of energy (i.e., amplitude) and timing measurements, highlighting the strengths and weaknesses of the proposed approaches. Moreover, to minimize the impact of jitter, the ToT technique will also be validated using a Charge Readout (CR) circuit. The circuits were implemented on a Printed Circuit Board (PCB) and interfaced to a Xilinx 28-nm Artix-7 200T Field Programmable Gate Arrays (FPGA) housed in a Nexys Video. In the FPGA, in addition to the logic for interfacing with the ADC for amplitude read-out, a TDC with a precision of 12 ps r.m.s. is also included. The TDC is responsible not only for generating the timestamp for the PD and INT circuits but also for estimating the energy in the ToT with and without the CF. It can be observed that it is possible to detect amplitudes up to some volts with a precision of a few millivolts using both time-based and voltagemode approaches at rates of tens of Msps maintaining a temporal resolution of tens of picoseconds.