High stability measurement system for the Olimpo IR calibration source

A specific on-board electronic system has been developed for driving the infra red source calibration of the balloon-borne Olimpo. This system operates at ambient temperature, whereas the source is located on the 2K stage of the cryostat. This electronic system first generates a heating signal on the iridium strip of the source, and then monitors the source temperature stability by several measurements of an NbSi thermometer which is located under the source. Priority in the design was to achieve a high stability measurement, with low heating perturbations of the source, and with a large temperature range of the electronics. The design merges a modulated bias system, with a low noise voltage differential amplifier, and a digital acquisition. The whole system is driven by a microcontroller whose algorithm was optimized to minimize the noise perturbations. Electronic components have been selected for their large temperature operation range, and for their low temperature sensitivity. Moreover, the electronic system is designed to compensate the thermal deviation of the components. The system has been tested successfully with simulated and real IR source for several temperatures. With 600 pW of total sensor dissipation injected during 40 ms, the measurement standard deviation achieved for the resistive value of a 10 kΩ thermometer reaches 0.5 Ω (48 ppm). Moreover the sensitivity of the electronic for the measurement is of 25 ppm/°C ranging from -60°C to +20°C, in agreement with the specifications.