The WaveCatcher Family of SCA-Based 12-Bit 3.2-GS/s Fast Digitizers
Résumé
The recent progresses in detector developments have raised the requirements for the associated readout electronics. High-end oscilloscopes are well adapted, but their cost per channel is high and their number of channels very limited. Precision test benches are also based on dedicated boards like Charge-to-Amplitude Converters, Constant Fraction Discriminators (CFD) and Time to Digital Converters (TDC). Performance can be very good, but most of these modules are expensive and house very few channels. Recently, alternative methods based on digital treatment of digitized detector signal have been developed. Such methods permit an easy calculation of the charge and amplitude, and achieve a timing resolution far better than the sampling frequency thanks to signal interpolation. Digitization systems have followed the progress of commercial ADCs, but the latter have drawbacks like their huge output data rate and power consumption. They also imply the use of very high-end FPGA to treat the data flux, thus introducing extra cost and complexity. Conversely, high speed Switch Capacitor Arrays (SCA) now offer sampling rates far above 1 GHz at low cost and low power consumption. Moreover, their companion FPGA can be low-end, thus also low-power and low-cost. The WaveCatcher board family has thus been developed for proposing an alternative to ADC-based digitizers (or even oscilloscopes). It is based on the SAMLONG SCAs developed since 2010 with the AMS CMOS 0.35-m technology. All boards offer a wide DC-coupled dynamic range of 2.5 V (with adjustable offset) coded over 12 bits, a bandwidth of 500 MHz and a sampling rate ranging between 3.2 and 0.4 GS/s, which allows them to finely sample high speed signals like very short pulses. The sampling depth is of 1024 samples. The number of channels goes from 2 for the USB-powered version packaged in a friendly plastic box to 64 (optionally 72) for the version housed in an autonomous mini-crate. Thanks to the servo-controlled matrix structure of SAMLONG and the care taken in the board design, the sampling time precision is of about 5 ps rms, which permits using the system as a high-resolution TDC between any pair of channels, as this precision remains constant between chips and boards. The boards offer a lot of functionalities, like threshold-triggering on any channel and numerous trigger modes including coincidence. All system versions house a 480-Mbits/s USB interface permitting an easy connection to PC and a good readout speed. In parallel to hardware and firmware developments, we developed a powerful and friendly acquisition software running on PC and transforming the latter into a 2 to 64-channel oscilloscope. It is currently used by most board users. An increasing number of labs or companies are now using the WaveCatcher boards worldwide on their test benches or physics experiments. Our next steps include expanding the number of channels while keeping the 5-ps time precision.