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This paper presents two mixed-signal monolithic gas sensor microsystems fabricated in standard 0.8- CMOS technology combined with post-CMOS micromachining to form the microhotplates. The on-chip microhotplates provide very high temperatures (between 200 C and 400 C), which are necessary for the normal operation of metal–oxide sensing layers. The first microsystem has a single-ended architecture comprising a microhotplate (diameter of 300 ) and a digital proportional-integral-derivative (PID) microhotplate temperature controller. The second microsystem has a fully-differential architecture comprising an array of three microhotplates (diameter of 100 ) and three digital PID microhotplate temperature controllers (one controller per microhotplate). The on-chip digital PID temperature controllers can accurately adjust the microhotplate temperatures up to 400 C with a resolution of 2 C. Further, both microsystems feature on-chip logarithmic converters for the readout of the metal–oxide resistors (which cover a measurement range between 1 and 10 ), 10-bit A/D converters, anti-aliasing filters, 10-bit D/A converters, serial interfaces, and bulk-chip temperature sensors. Carbon monoxide (CO) concentrations in the sub-parts-per-million (ppm) range are detectable, and a resolution of 0.2 ppm CO has been achieved.