SE S ORS ACTWORS A E LS EVI E R Sensors and Actuators A 52 (1996) 198-202 PHYSICAl. Design of integrated thermal flow sensors using thermal sigma-delta modulation Huibert-Jan Verhoeven, Johan H. Huijsing Electronic Instrumentation Laboratory/DIMES, Faculty of Electrical Engineering, Delft University of Technology, PO Box 5031, 2600 GA, Delft, Netherlands Abstract Large-scale use of thermal flow sensors in consumer electronics, medical and automotive applications is seriously hampered by the limited response speed and the non-standardized format of the output signal of conventional thermal flow sensors. A thermal sigma--delta-based feedback system can be used to provide an optimal interface for a wide variety of new as well as existing thermal flow sensors. Advantages of this system are an improved response time of the sensor system and an inherent analog-to-digital conversion of the measurement signal. Realized sensor implementations range from sensors for flow control in consumer electronics, respiration control, to industrial mass-flow control. Keywords: Flow sensors; Thermal sensors; Smart sensors; Interface electronics 1. Introduction Realizing both functional and economically feasible inte- grated combinations of sensor functions and signal-process- ing electronics puts a number of constraints on the complexity and component tolerance of the on-chip circuits. In particular, the analog-to-digital (A/D) conversion techniques available are a limiting factor. Most conventional A/D converters are impractical to incorporate on a sensor chip, either due to excessive die-area consumption or because of high demands on on-chip component accuracy. The simplicity and rugged- ness of sigma-delta A/D converters makes this category of converters an excellent candidate for smart-sensor applica- tions [ 1,2]. A number of sensor principles can be successfully merged with the A/D conversion function. By entering the sensor in the feedback loop of a conventional sigma--delta converter, sensor response speed and linearity can be greatly enhanced, while at the same time obtaining a pulse-rate-modulated out- put signal. Implementations of this technique have been reported in the field of electrostatically driven accelerometers [3]. In this paper two thermal flow sensors are presented, which are part of a so-called thermal sigma-delta configuration. As part of the signal processing takes place in the thermal signal domain, the complexity of the required electronic circuits is very low and can be realized in every commercial bipolar of 0924-4247/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD10924-4247(95)01 100-5 CMOS process. A pulse-rate output signal with an eight-bit accuracy can, if necessary, be obtained within 10 ms, which opens a number of new fields for which the response speed of conventional thermal flow sensors [4] was previously considered to be insufficient. As a typical example a smart thermal flow sensor suited for respiration monitoring and control is described. To illustrate the wide range of imple- mentations, a smart thermal flow sensor with a complete I2C bus interface for low-cost domestic applications based on thermal sigma-delta modulation is also presented here. 2. Thermal sigma-delta modulation Conventional integrated thermal flow sensors operate in a constant-power mode. The flow-induced on-chip temperature differences are directly related to the flow speed. Low response speed is inherent to the highly limited bandwidth of most integrated thermal systems. Attempts have been made to increase the response speed by applying thermal feedback [ 5 ]. The maximum obtainable loop gain of the sensor system is limited by the higher-order character of the sensor, as well as by the limited bandwidth of the amplifier. Fig. 1 shows a block diagram and a root locus of a thermal flow sensor with analog feedback. The sensor is represented by a second-order system. In practical