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 mu m CMOS process. Measured results show that when measuring resistance, the input-referred resistance resolution is 10.5 Omega for 100 Hz integration bandwidth. Using an off-chip thermistor, the readout circuit covers a temperature range of 0-75 C-degrees and achieves an equivalent temperature resolution of 16.4 mK(rms). In current mode, the readout circuit has an input range of 0.5 mu A 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 mu V-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 mu W 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.

Automated summary

In this article, a frequency-locked loop (FLL) based multimodal readout integrated circuit (IC) is presented for interfacing with off-chip temperature, electrochemical, and pH sensors. The readout circuit can measure resistance, current, and voltage without additional active analog front-end circuits. Experimental results demonstrate the performance of the circuit in different measurement modes.