A new front-end readout circuit design using an adaptive amplifier is proposed by this study for magnetic and optical scales. The purpose of the integrated chip (IC) designed herein is to provide the magnetic/optical scales the capability of sensing translational or angular positions of an object in largely-varying amplitudes with high resolutions. Toward this end, a new front-end readout circuit is designed with two adaptive programmable gain amplifiers (PGAs) and two analog-to-digital converters (ADCs). The PGA is particularly designed to accept signals up to 1 MHz from MR sensor signals, which could be one of three voltage levels: 120 mVp-p, 250 mVp-p, 330 mVp-p. The resulted power consumption is 56 mW. The chip also includes a successive approximation register (SAR) ADC having two 12-bit channels for sine-to-digital conversions. The linearity of ADC achieve satisfactory performance (±1LSB) in the input range of 0.5∼2.5V in Vp-p. The chip was fabricated by the TSMC 0.35-micron 2P4M CMOS technology for verification.