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High Temperature, Time Domain Sensor Interface based on Injection Locked Oscillators as Phase Shifters
Keywords: High temperature, injection locked oscillators, phase shifter
Many demanding applications such as engine control system, well logging or aeronautic require sensing systems with a high temperature range of operation. A sensing system is typically composed of a transducer and a sensor interface that should be kept as close as possible to improve the sensing system performances whatever the temperature of operation is. Besides, these sensing systems would benefit from silicon integration that could help to reduce the occupied volume, to increase reliability and, to allow more complex and precise functions. However, todays integrated sensing systems have a limited temperature range due to the sensitivity of the constitutive blocs to environmental variations and among them to the temperature. To address this issue, several hardening techniques have been proposed. Among them, the local hardening to reduce the temperature sensitivity of each bloc of the circuit separately [1][2] and, the use of closed loop architectures [3]. However, the beneficial impact of these techniques is limited due to the reliability degradation or the leakage current increase at high temperature operation or during temperature- cycling. Due to these limitations, a new solution that represents the signal in the time domain has emerged [4][5]. Since time-domain signals have higher noise margin, this approach is susceptible to lead to a low temperature dependence of integrated sensing systems. This paper presents a novel interface architecture for resistive sensors working in time-domain and taking advantage of the properties of an Injection Locked Oscillator (ILO) with an operational temperature range that can be extended over 290C. The most interesting property of these oscillators is that once locked to their synchronization input, the phase shift between the synchronization signal and the oscillator output signal is a function of the difference between the injected signal frequency and the oscillator free running frequency [6] [7]. Since the phase shift does not generally depend on the variation of the signal frequency, it would be interesting to use ILOs to perform the analog-to-time-conversion to represent the signal delivered by the sensor in the phase domain. Therefore, this approach has the advantage of releasing the constraint on using a thermally stable frequency and guarantees more stable performances over the temperature compared to conventional analog-to-time conversion based architecture. Besides, the sensor interface is designed in a way that makes it only dependent on the relative accuracy of its parameters, which lead to a high robustness to the temperature variation. The circuit is mainly composed of two ILOs controlled by differential resistive sensor. The differential topology is adapted to enhance the sensing system stability. The sensor analog output Vs will respectively increase and decrease the free running oscillation frequency of respectively the first and the second ILO. Consequently, the phase shift between the two ILOs to the respect of synchronization signal will vary symmetrically. Therefore, the measure of the sensing system is defined by the phase shift between the two ILOs outputs, which is digitalized by a high frequency counter. The clock frequency of this latter is a multiple of the synchronization frequency and consequently the sensor interface system is wholly synchronous with the synchronization signal. Thus, the sensor interface output depends only on the relative accuracy of its parameters rather than absolute variations. Therefore, a low sensitivity to the temperature variation is achieved. The circuit is designed using a XFab 0.18m partially depleted silicon on insulator technology. Simulations show that the variation of the sensor interface output is below 0.5percentage for temperature ranging from 40C to 250C. The circuit is currently under test, measurement results will be added in the final version of the paper.
Grenoble, Rhne Alpes

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