A digitally configurable reference capacitance with mismatch compensation for biosensor readout circuits

The most popular capacitance biosensor readout circuits are based on charge sensitive amplifiers. High precision results can be obtained if the difference between the biosensor and a reference capacitance is estimated but this is difficult to be achieved if the range of the biosensor capacitance is wide. In this paper, a novel algorithm implemented in hardware that is used to configure an appropriate reference capacitance close to the biosensor value is presented. The resulting readout circuit is capable of accurately monitoring the changes of a biosensor capacitance, regardless of its initial value. More specifically, the reference capacitance is automatically configured to a value between 0 and 630pF in steps of 10pF. The proposed system can be used to measure an absolute biosensor capacitance of up to 640pF with 2.5fF sensitivity. The 2.5fF resolution is achieved by using a 12-bit ADC instead of a 18-bit ADC that would be required to measure the absolute biosensor capacitance value with the same accuracy. In this way, a lower area/power and higher accuracy readout system can be designed. Of course the capacitance ranges and resolution can be easily adapted to different application requirements.