Design of an Energy-Efficient Current-to-Frequency Converter for a Wearable Sensor Platform

This paper presents the design of the ANTIGONE ASIC in AMS 0.35 μm technology that interfaces with ion-sensitive field effect transistors (ISFETs) to detect biomarkers like pH, Na+ and K+ in human sweat. The ISFET sensors generate a low current dc signal, so a current-to-frequency converter (CFC) architecture is used to measure the produced current of the different sensors in time multiplexing. The ASIC has to properly bias the ISFETs and send the digitized output signal to a microcontroller through an SPI interface. In the front-end, a switched capacitor circuit is used to balance the charge that is stored in the current integrator. The 12-bit converter is designed for low power consumption. The weak inversion region of operation is exploited to achieve the highest gain for the same power budget. The different analog blocks are following a power activation sequence in order to limit the power consumption during the idle states of the system.