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Miniaturized calorimetric flow sensors suffer from a limited measurement range if operated with constant heating power. To overcome this drawback, we utilize a thermal sigma-delta modulator to achieve a constant average excess temperature of the sensor membrane. The duty cycle of the modulator output fully covers the flow information. This novel method exhibits the advantage that the slope of the output characteristic and hence the flow sensitivity can be optimized for a certain flow range by choosing an appropriate amplitude of the heating voltage. Finally, the influence of the clock frequency on the system accuracy was investigated by extensive SPICE modeling of the thermal system.