High Gain Capacitance Sensor Interface for the Monitoring of Cell Volume Growth

CMOS-integrated capacitance based cell sensing has shown promising results for the monitoring cell volume growth. The process utilizes the direct correlation between cell health and measured capacitance. Prior sensors have used charge based capacitance measurement, which has limitations in offset correction, range and resolution. An alternative approach to monitoring small changes in the electrode capacitance is to measure the frequency change of a capacitance controlled ring oscillator containing a sensing electrode, that acts as a variable capacitor. This high gain sensor design uses a differential measurement, with which the offsets can be calibrated out. In addition, the resolution of this sensor is limited by the amount of read time given to each pixel. For an oscillation frequency of 24 MHz, the sensors are expected to achieve a resolution less than 10 aF. The expected sensitivity of the overall system is 1.33 mV/aF. The sensor has been incorporated into a 3x4 array prototype in 130 nm CMOS process which will be packaged for operation in cell culture