A new miniaturized, non-dispersive, infrared (NDIR) sensor for CO2 intended to be installed in mobile phones and its drive/readout circuits are presented in this study. A typical NDIR sensor consists of three main components; an infrared (IR) lightemitter (light source), a gas chamber, a photo detector (PD) light receiver) and the associated drive/readout circuits. The geometry of the gas chamber is optimized to minimize the total module size to approximately 10 mm x 5 mm x 5 mm, which is much smaller than commercially-available gas sensors. Driver and readout circuits are successfully designed and taped out. The driver circuit intends to generate pulse width modulation (PWM) signal to control proper dimming of LED. The readout circuit, which acquires small signal from photo detector then converts to digital values, includes amplifier, low pass filter and analog-todigital converter (ADC). The proposed circuit is fabricated by the TSMC 0.35-μm CMOS process, where the area is 4.527 mm2 while power consumption is 60.16 mW for the whole chip. The resolution is less than 12 ppm along with time constant is 0.1 sec.