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

In this article, a frequency-locked loop (FLL) based multimodal readout integrated circuit (IC) for interfacing with off-chip temperature, electrochemical, and pH sensors is presented. By reconfiguring its switched-capacitor feedback network, the readout circuit is able to measure resistance, current, and voltage without additional active analog front-end circuits. A prototype IC was fabricated in a 0.18 μm CMOS process. Measured results show that when measuring resistance, the input-referred resistance resolution is 10.5 Ω for 100 Hz integration bandwidth. Using an off-chip thermistor, the readout circuit covers a temperature range of 0-75 ^°C and achieves an equivalent temperature resolution of 16.4 mK_rms. In current mode, the readout circuit has an input range of 0.5μA and an input-referred current noise as low as 40.6 pA_rms for 100 Hz bandwidth. Interfacing with an on-chip potentiostat, glucose chronoamperometry is demonstrated. In voltage mode, a minimum input-referred voltage noise of 31.7 μV_rms is achieved, and the IC can measure a pH range from 1.6 to 12 using a commercial pH probe. At a 1.2 V supply, power consumption of the readout circuit is below 10 μW for all three measurement modes. Additionally, the prototype IC includes an integrated wireless transmitter that implements on-off keying modulation, and a wireless multimodal sensing system utilizing the FLL-based readout circuit is demonstrated.