A biosensor readout circuit with 3fF resolution and broad configurable range

Abstract

Capacitance to digital conversion is often used for biosensor readout circuits based on charge sensitive amplifier Front Ends. Absolute capacitance can be measured by the time it takes to discharge a capacitance through a current source. Higher precision is obtained, if the difference between the unknown capacitance and a reference one is estimated but this is difficult to be achieved if the range of the biosensor capacitance is broad. In this paper, an absolute capacitance readout circuit appropriate for use with capacitive biosensors is presented. It can cover a capacitance range of 180pF with an experimentally measured capacitance accuracy of 3fF, close to the 2.44fF theoretical resolution (Integral Non Linearity-INL<1 LSB). This mixed signal readout circuit has been implemented and successfully tested in 90nm TSMC CMOS technology occupying 0.04mm2 of die area. This system is extended for a range of 640pF based on a method that digitally configures an appropriate external reference capacitance close to the unknown capacitance value. The 3fF resolution is achieved by using a 12bit instead of 18-bit ADC that would be required to measure an absolute capacitance value with the same accuracy.