Modeling and simulation of thermal sigma-delta modulators

This paper describes a new methodology for the simulation of thermal sigma-delta modulators. Its essential feature is the derivation of a compact electrical model of the modulator´s thermal loop filter from the filter´s measured step response. This model can then be used to simulate the entire modulator in the electrical domain. To verify this methodology, the behaviour of an existing modulator, used to read-out a thermal flow sensor, was studied. The sensor is incorporated in the feedback loop of the modulator, where it functions as a thermal low-pass filter. A compact model of this filter was derived from the sensor´s measured step response. Using this model, the modulator was then simulated in SPICE. The simulation results correspond well with the modulator´s actual behaviour.