CMOS interface of a flow sensor for urodynamic monitoring

A signal conditioning CMOS integrated circuit is proposed for use with a micro-machined resistive flow sensor, adopted in order to non-invasively monitor urinary dysfunctions in male patients. The circuit is based on a relaxation oscillator with duty-cycle modulation and relative A/D conversion at chip level. The implemented resistive bridge-to-duty-cycle converter produces an output signal with duty-cycle highly linear in the change in resistance of the flow sensor´s elements. The range of variability of the sensor resistors is a few hundreds of ohms in typical applications in the field of urology. Experimental tests with 1% precision resistors carried on the sensor interface evidenced a maximum non-linearity below 0.9% and a resolution of almost 7 bits, with a minimum measurable change in resistance less than 2 Ω over the tested range. The proposed circuit, provided with digital output, also allows simple signal interfacing to any standard PC for data transfer and storage.