Thermophile-based flow sensors can be realized using polysilicon-aluminium thermocouples and polysilicon as heating resistor. In differntial structures the temperature difference deltaT between the two thermopiles is proportional to the flow velocity. DeltaT not only depends on the rate of air flow, but also on the power dissipated by the heating resistor. It is thus importnat that the heating power is independent of the ambinet temperature and supply voltqage variations to achieve good linearity for the flow sensor.
Polysilicon resistors show relatively high temperature coefficients in the order 0.1% per Kelvin. For a 100K temperature range the change in resistance and heatingpower is in the order of 10% which is too large for most applciations.
An effective way of reducing this temperature dependence is to connect a series of resistor of the same value and drive the two resistors with a stabilized voltage. It can be shown that the same resistance change of 10% now causes a heating power change of only 0.25%, provided that the compensation resistor can bekept cold. As half the power is dissipated in the compensation resistor, the sensitivity of the sensor is reduced, A more elaborate scheme with the same compensation characteristic but nominally no power dissipation in the compensation resistor solves this problem. This version has been integrated as a functional prototype and successully tested.