An Energy-Efficient Miniaturized Intracranial Pressure Monitoring System

Abstract

A miniaturized pressure-sensing microsystem targeting intracranial pressure monitoring is presented. The system takes full advantage of Invensense MEMS-CMOS process to heterogeneously integrate the sensor and interface. This integration type requires no fabrication postprocessing and results in sub-pF sensor-interface parasitic interconnection capacitance C, which is an order of magnitude smaller than previously reported C,s. Since energy efficiency is of main concern, the minimum energy consumption for maintaining a certain signal-to-noise ratio (SNR) is analytically calculated and compared for two energy-efficient sensor front ends, namely, the switched-capacitor (SC) capacitance-to-voltage converter (CVC) and the successive approximation register (SAR) capacitance-to-digital converter (CDC). The comparison reveals for small values of C, and for low-to-moderate SNR, the SAR CDC outperforms the SC CVC in terms of power consumption. Heterogeneous integration of sensor and CMOS electronics results in only 720 fF of C, which enables direct SAR capacitance-to-digital conversion. Correlated double sampling is also integrated into the proposed SAR switching scheme to combat 1/f noise and the input-referred offset voltage of the comparator. The entire pressure-sensing system measures 2.2 × 2.6 × 0.4 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> in size, consumes 130 nW at 60-Hz sampling rate, and obtains 57-dB SNR with 0.2% sensor-electronics combined linearity over 520-mmHg pressure range.