A low-power minaturised intracranial pressure monitoring microsystem

Area and available energy are two opposing factors in the design of implantable biosensors, e.g. neural stimulators and bio-fluidic pressure sensing systems. On the one hand, the device should be small enough to be seamlessly assimilated to the body environment. On the other hand, the available power sourced by an on-board battery drops proportionally with the size of the implant. Moreover, implantable devices are preferred to operate on a battery-free mode for leaking toxic chemicals of the battery may result in life threatening health conditions. Also, the life-time of the implant will be determined by that of the battery whose extension demands battery replacement by means of costly surgical procedures. Thus, other energy sources such as RF power scavengers or fuel cells are considered an optimal solution as power sources. In such cases, however, the inverse relationship between the available power and the size of the implant is more pronounced making design of chronic non-battery-operated implantable systems one of the most stringent of engineering problems. The ultimate goal of this work is realisation of a fully implantable chronic intracranial pressure (ICP) monitoring system. Due to the required mm-scale form factor of the implantable device, the available power is scarce. This calls for investigation of new circuit and sensor integration techniques to decrease the total power consumption of the system down to a few hundreds of nano watts. So the main focus of this work is design of an ultra-low power integrated circuit (IC) for measuring ICP. Power consumption minimization of the sensing system proposed in this work paves the way for integration of an RF-power scavenger or biological fuel cells. The proposed sensing system also takes full advantage of Invensense MEMS-CMOS process to heterogeneously integrate the sensor and interface. This integration type requires no post-processing and results in sub-pF sensor-interface parasitic interconnection capacitance Cp which is an order of magnitude smaller than previously reported Cp’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 Cp and for low-to-moderate SNRs, the SAR CDC outperforms the SC CVC in terms of power consumption. Heterogeneous integration of sensor and CMOS electronics results in only 720fF of Cp which enables direct SAR capacitance to digital conversion. Correlated double sampling (CDS) is also integrated into the proposed SAR switching scheme to combat 1/f noise and the input referred offset voltage of the comparator. The proposed system was fabricated in Global Foundaries 0.18μm CMOS process and the entire pressure sensing system measures 2.2× 2.6× 0.4mm3 in size, consumes 130nW at 650Hz sampling rate and performs 12-bit digitization with >0.2% sensor-electronics combined non-linearity over 520mmHg pressure range.