A 1.76 mW, 355-fps, Electrical Impedance Tomography System With a Simple Time-to-Digital Impedance Readout for Fast Neonatal Lung Imaging

Functional lung imaging for neonates using electrical impedance tomography (EIT) requires a fast frame rate to avoid under sampling the faster respiratory rate in neonates than adults. Also, the EIT system must have low power consumption to facilitate wireless operation during kangaroo care. The application of existing EIT systems primarily designed for adults is limited by the trade-off between power consumption and speed of the impedance readout. This article presents an integrated EIT system in a 65 nm CMOS node featuring a novel ultralow-power time-to-digital impedance readout method based on dc cross-point detection. To increase the resolution of the impedance readout, a dynamic comparator with multi-phase voltage-controlled oscillator (VCO)-based least-common-multiple (LCM) coherent clock control is used, yielding a measured impedance error of 0.94% and a phase error of 0.81° over a frequency range from 100 to 500 kHz in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10~\mu $ </tex-math></inline-formula>s measurement time. In addition, a novel fast-settling switch matrix allows multiplexing the single impedance readout channel, significantly decreasing power consumption without degradation in speed. The EIT system has a maximum frame rate of 355 frames per second (fps) and a total power consumption of 1.76 mW. The impedance readout and digitization consume <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$172~\mu $ </tex-math></inline-formula>W, a power reduction of 83%–93% compared to prior work. The frame rate is both suitable for neonatal lung imaging and other fast EIT applications. Both in vitro and adult in vivo measurements are presented.