Analog-Signal Quality Characterization of the FLITES Distributed 192-Channel Data Acquisition System

The Fibre-Laser Imaging of gas Turbine Exhaust Species (FLITES) project continues to target chemical species tomography for quantification and video-rate imaging of Carbon Dioxide within the exhaust plumes of high-thrust aero-jet engines. This remit, and the scalability needed to i) increase imaging speeds and resolutions and ii) extend it towards other pertinent gases, have been iteratively tackled through custom mechanical, optical and electronic designs targeting a commercial engine testing facility. We previously published work towards a high-speed (40MS/s/channel 14-bit) and multichannel (192-input) fully-parallel data acquisition (DAQ) system, utilizing Ethernet connectivity for
remote operation and field-programmable gate array (FPGA)-based digital lock-in amplification (DLIA). The data rates are significant, 8.96Gb/s for a single printed circuit board (PCB) and over 107.5Gb/s for the 12-node, full-system. The management of sixteen (560Mb/s) low-voltage differential-signalling
(LVDS) dual-data-rate (DDR) lines per FPGA has presented significant challenges, which we discuss here in overview. However, for the accurate analysis of gas concentrations, using tunable diode-laser absorption-spectroscopy (TDLAS) and wavelength-modulation spectroscopy (WMS), the analog performance is of interest and will be discussed. Specifically, while a 1st order 10Hz high-pass and 2nd order 3MHz low-pass response have been previously confirmed, we achieve a best-case signal-tonoise ratio (SNR) of 55.76dB, a spurious-free dynamic range (SFDR) of 50.8dB and a thermal noise floor of -60dB. The inputs are shown to be linear over the range of 0-1600mV (2Vpp ADC range) and the limiting factor is found to be harmonic distortion induced by single-sample glitches that are possibly linked to high-frequency simultaneous switching noise.