A Flow-to-Digital Converter based on a Thermal-Sigma Delta Modulator

Abstract

Flow sensing is essential in various industrial, commercial, and biomedical applications. Although many types of flow sensors exist, thermal flow sensors are widely used due to their simple sensing elements (heaters and temperature sensors) and low fabrication costs. A low-cost interface for thermal flow sensors can then be realized with a Thermal Sigma-Delta Modulator (TΣΔM), which regulates the temperature of the sensing element, while simultaneously digitizing the required power.
This thesis discusses the design of a pseudo-differential TΣΔM to read out a thermal flow sensor, which consists of two heating resistors in thermal contact with the flow. The modulator maintains both resistors at a constant temperature and digitizes the required differential power. The resistors have a Positive Temperature Coefficient (PTC) and can thus be used as heaters and temperature sensors in a time-multiplexed manner, resulting in a compact realization. A prototype was fabricated using discrete components on a PCB. With a sampling frequency of 5kHz, the modulator achieved a thermal noise-limited resolution of greater than 13 bits in a bandwidth of 1.5Hz for a full-scale input of 7mL/min. The entire read-out was realized with two integrators, two clocked comparators, and a few switches, resulting in a simple, low-cost interface with direct digital read-out.