A new trend in the development of integrated sensor is to include the signal conditioning circuit and sensor control circuit on the same chip. This approach can improve the performance and capability of the sensor. Several successful applications have been developed for pressure sensors and accelerometers. However most reported tin oxide gas sensor still at discrete level. To make the monolithic integrated gas sensor available in the future, integration of the front-end circuit should be considered....[ Read more ]

A new trend in the development of integrated sensor is to include the signal conditioning circuit and sensor control circuit on the same chip. This approach can improve the performance and capability of the sensor. Several successful applications have been developed for pressure sensors and accelerometers. However most reported tin oxide gas sensor still at discrete level. To make the monolithic integrated gas sensor available in the future, integration of the front-end circuit should be considered.

To control the tin oxide gas sensor, the heating power is an important issue to consider. The heating source is usually provided by passing a current through a polysilicon resistor. It is well known that polysilicon have relatively high temperature coefficient. The heating power cannot be controlled precisely with this simple arrangement. A resistor matching scheme is proposed in [ 11, 12, 13]. However the proposed scheme can only be compensated at a fix temperature. In this thesis, a new power control circuit is presented. With the new circuit technique, called the Bipolar-like translinear loop, the heating power can be compensated for wide range of heating temperature. The power control circuit can control the heating power to within 1% for a ±50% change in heating resistor value. The heating power can be adjusted by changing the reference current of the circuit. Compared with the heater without compensation, the change in heating power can be reduced by 50 times.

A signal conditioning circuit is also considered in this thesis. Tin oxide gas sensor is a chemo-resistor device. The resistance can change by as much as 10 times due to change in gas concentration. This gives rise to large charge in the chemo-resistance. The commercial method for the signal conditioning is to use the potential divider. This technique is not suitable when the output resistance changes is large. We proposed a new signal conditioning circuit. The system can detect gas concentration as low as 30ppm. The lowest gas concentration is limited by our measurement system.