A CMOS SWITCHED CAPACITOR FILTER BASED POTENTIOMETRIC READOUT CIRCUIT FOR CHEMFET SENSOR

Abstract

There are wide ranges of potentiometric sensors available for chemical sensing. The latest implementation of these sensors is CHEM resistors. In real time environment for an in-vivo application area, noise and power are the important factors. Though the selectivity of CHEM resistors improved with recent technological developments analog front end measurement circuits are always the limiting factor. Without compromising the performance parameters like gain and op-swing it is a challenge to improve the noise and power. Noise and power are the two factors which always require improvement. Also, there is trade-off between noise and power which needs to be balanced. The fundamental noise generated by the device is a challenge to handle. CMOS amplifiers usage is limited by the non-deterministic noise like flicker, short and thermal. It is seen that for potentiometric circuit the CDS is preferable for the sampled data circuits like the switched capacitor so that the base bad noise behavior is not made worse by noise aliasing. This eliminates the DC offset and drifts. Though we use CDS for various potentiometric measurements the filtering stage after the CDS limits the capability of noise reduction. Therefore in this thesis work the detailed analysis of CDS with switched capacitor based LP filter is introduced. The architecture used helps in accurate measurement of pH, where the ratio of the chemical compound like sodium, potassium and chlorine is very critical. The integrated circuits are designed using 0.18μm CMOS process. The layout of the designed chip is 3.8mm × 3.8mm. The design simulation is done with a load of 10pF and 100Hz input signal. The circuit operates with a supply range ±1.65V linearly reproduces the sensor output voltage ±1.5V. The noise measured with 1MHz sampling clock is 0.683μVrms and the power required for the system is 124.1μW.