High-time-resolution X-ray observations of compact objects provide direct access to strong-field gravity, to the equation of state of ultradense matter and to black hole masses and spins. A 10 m²-class instrument in combination with good spectral resolution is required to exploit the relevant diagnostics and answer two of the fundamental questions of the European Space Agency (ESA) Cosmic Vision Theme “Matter under extreme conditions”, namely: does matter orbiting close to the event horizon follow the predictions of general relativity? What is the equation of state of matter in neutron stars? The Large Observatory For X-ray Timing (LOFT), selected by ESA as one of the four Cosmic Vision M3 candidate missions to undergo an assessment phase, will revolutionise the study of collapsed objects in our galaxy and of the brightest supermassive black holes in active galactic nuclei. Thanks to an innovative design and the development of large-area monolithic silicon drift detectors, the Large Area Detector (LAD) on board LOFT will achieve an effective area of ~12 m² (more than an order of magnitude larger than any spaceborne predecessor) in the 2–30 keV range (up to 50 keV in expanded mode), yet still fits a conventional platform and small/medium-class launcher. With this large area and a spectral resolution of <260 eV, LOFT will yield unprecedented information on strongly curved spacetimes and matter under extreme conditions of pressure and magnetic field strength.

In this paper, we present the design and the characterization of a wide-dynamic-range interface circuit for resistive gas-sensors able to operate without calibration. The circuit is based on resistance-to-frequency conversion, which guarantees low complexity. The state-of-the-art of this measurement method has been improved first by separating the resistance value controlled oscillator circuit (RCO) from the sensing device, thus leading to higher linearity performance, and then by exploiting a novel digital frequency measurement system. Measurement results on a silicon prototype, designed in a 0.35-mum CMOS technology, show that the circuit achieves, without calibration, a precision in resistance measurement of 0.4% over a range of 4 decades and better than 0.8% over 5 decades (dynamic range, DR = 141 dB). Furthermore, after calibration, it reaches a precision of 0.4% for resistance values ranging between 1 kOmega and 1 GOmega, thus leading to a DR of 168 dB. The prototype chip consumes less than 15 mW from a 3.3-V supply.

In this paper an integrated wide-dynamic-range interface circuit for resistive gas-sensors arrays is presented. The proposed device consists of a multiscale transresistance continuous time amplifier followed by a 13-bit incremental A/D converter. The circuit selects automatically the scale to use for each measurement and includes two digital-to-analog converters for the calibration of offset and gain of each scale. The proposed interface circuit achieves a measurement accuracy almost always better than 0.1 % over a sensor resistance range of more than 5 decades [100 Omega-20 MOmega], leading to an equivalent dynamic range of about 160 dB. The chip has been realized with a 0.35 mum CMOS technology and occupies an area of 3.1 mm² consuming 6 mW from a 3.3 V power supply.

The LOFT mission concept is one of four candidates selected by ESA for the M3 launch opportunity as Medium Size missions of the Cosmic Vision programme. The launch window is currently planned for between 2022 and 2024. LOFT is designed to exploit the diagnostics of rapid X-ray flux and spectral variability that directly probe the motion of matter down to distances very close to black holes and neutron stars, as well as the physical state of ultradense matter. These primary science goals will be addressed by a payload composed of a Large Area Detector (LAD) and a Wide Field Monitor (WFM). The LAD is a collimated (<1 degree field of view) experiment operating in the energy range 2-50 keV, with a 10 m² peak effective area and an energy resolution of 260 eV at 6 keV. The WFM will operate in the same energy range as the LAD, enabling simultaneous monitoring of a few-steradian wide field of view, with an angular resolution of <5 arcmin. The LAD and WFM experiments will allow us to investigate variability from submillisecond QPO’s to yearlong transient outbursts. In this paper we report the current status of the project.