Oscilllator synchronisation problem in a BiCMOS analog-digital integrated circuit dedicated to a capacitive pressure sensor

Primé à la présentation :"outstanding paper award"; International audience; A new type of hybrid pressure sensor has been realised using silicon/Pyrex capacitive sensing cell and a digital output BiCMOS transducer. A ratiometric scheme has been used to self-compensate thermal drifts and nonlinearities. Two demonstrators have been designed, mounted and tested. The first one, implemented with four chips to evaluate the feasibility, is characterised by a relative sensitivity close to-1.85%/bar, a nonlinearity in the order of 1.1%FS and an offset thermal coefficient smaller than 20 ppm/°C. A simple analytical model specifies that stray capacitors induce a decrease in sensitivity and a nonlinearity increase. The second demonstrator has been implemented with two integrated chips. It consists, on the one hand, of two pairs of converters and counters integrated in the same chip, and on the other hand, of a reference and measurement capacitors into the same sensing cell. This configuration reduces stray capacitors by a factor two but provokes interference and synchronisation phenomena. Characterisation and PSPICE modelling have permitted to understand, to localise causes and then to propose some solutions to avoid or minimise these problems.