In this paper, a CMOS multi-sensor readout circuit is presented. A multiple differential-input operational amplifier (MDI-OPA) with three distinct positive inputs and one common negative input is designed to take some one of three inputs to act as a general differential-input OPA by a built-in multiplexed architecture. A voltage-to-current converter and a current-controlled oscillator are integrated with the MDI-OPA in order that the analog voltages from sensors can be selectively multiplexed to generate pulse outputs which frequencies are linearly proportional to the related input voltage. A current-offset structure is added into the oscillator to shift the transfer characteristic line of the output frequency versus input voltage, which usually varies due to process variation. The measured output transfer characteristics of three input channels show nearly the same sensitivity of 90 Hz/mV or so with linearity of 99.99% at least by the assistance of the current-offset mechanism.