Abstract:
The pressure at different organs is highly regulated in the human body, which is a key indicator
of health in many chronic conditions. Monitoring these pressures remotely and precisely
provides useful information for managing health and treating disease. To measure these
pressures, less invasive and miniaturized implantable devices are required. Creating highly
miniaturized pressure sensors would allow them to be deployed by minimally invasive methods
such as an injectable pressure sensor. However, creating injectable sensors requires highly
integrated pressure sensor signal conditioning circuits to be developed as current methods are
larger than those that can be included in an injectable device. Miniaturization to this level is
only possible through creating an ASIC which includes all functions to allow sensor readout,
power reception and data transfer. This work investigated a low-power, injectable pressure
sensor readout ASIC in a 180 nm CMOS process to support injectable capacitive pressure
sensors.
The ASIC was developed in a step-by-step process with three fabrication runs of XFAB XH018
CMOS which allowed subcircuits to be tested throughout the development and to achieve the
design of a pressure sensor readout ASIC. The first ASIC, a capacitance to frequency converter
ASIC was designed, fabricated and tested which confirmed the function of 1.8V LDO, bandgap,
oscillator and VCO circuits with a wirelessly powered PCB coil. Then, a second ASIC was
designed to improve the accuracy of the first ASIC using a new switched-capacitor (SC) analog
front end (AFE) and a SC single-slope ADC with an inductive wireless power supply and load
shift keying (LSK) for a wireless data transfer, which was also fabricated and tested. It
confirmed the function of the SC AFE (capacitance to voltage), I2C, 500 kHz RC oscillator,
2.5V HV LDO and LSK circuits.
The third ASIC corrected an issue identified in the second ASIC testing where logic errors lead
to corruption of the digital data in the single-slope ADC. Finally, a complete system ASIC that
met all requirements was designed and fabricated with the major addition being the
incorporation of a 19-bit capacitance to digital converter utilizing a Δ∑ADC. The ASIC was
evaluated for accuracy and power consumption in a custom-designed bench top rig which
incorporated a pressure-controlled environment, a wireless power transfer system and a data
receiver implemented in a Field Programmable Gate Array (FPGA). This confirmed the ASIC
met the technical requirements of injectable pressure sensors.
In summary, three different types of pressure sensor readout ASIC were fabricated and tested.
The final ASIC had a 265 μW power consumption at 1.8 V power supply and 575–900 mmHg
absolute measuring range with a better than 1 mmHg accuracy. This ASIC is a key step towards
the development of high precision wireless injectable pressure sensors for applications such as
Intracranial Pressure (ICP), compartment and cardiovascular pressure monitoring.