A Reconfigurable Tri-Mode Frequency-Locked Loop Readout Circuit for Biosensor Interfaces

In this work, a reconfigurable frequency-locked loop (FLL) readout integrated circuit (IC) is presented for interfacing with off-chip temperature, pH, and electrochemical transducers. It measures resistance, current, and voltage without additional active analog front-end circuits. In temperature or pH mode, off-chip thermistor resistance or pH probe output voltage is linearly converted to FLL output period. In electrochemical mode, the current of a on-chip potentiostat is linearly converted to FLL output frequency. Period and frequency are quantized by an oversampling time-to-digital converter at 3 MHz. A prototype IC was fabricated in a 0.18 µm CMOS process. Measured results include a temperature resolution of 16.4 mK at 100 Hz bandwidth, and a linear pH range from 4 to 10. Electrochemical sensing has a maximum current range of 240 µA and is demonstrated using glucose chronoamperometry. At a 1.2 V supply, power consumption of the multi-sensor FLL readout circuit is 9 µW.