A Direct Conversion Transceiver for Portable Microfluidic NMR Flowmeters

This paper presents a direct conversion transceiver intended for use in a microfluidic NMR flowmeter. It consists of an H-bridge power amplifier, which drives a hand-wound milimeter-sized coil with RF signals, and a direct conversion receiver, which amplifies the NMR signals picked up by the coil, and then digitizes them with an asynchronous 8 bit SAR ADC sampling at 70 MHz. Fabricated in a 65 nm CMOS technology, the receiver achieves a noise spectral density of $1 \mathrm{nV} / \mathrm{sqrt}(\mathrm{Hz})$ at 21 MHz, while dissipating only 36 mW. A microfluidic flowmeter based on the transceiver and a handheld 0.5 T permanent magnet can measure flow rates up to $96 \mathrm{ml} / \mathrm{h}$ in a 0.8 mm inner-diameter channel with $\pm 1.3 \%$ full-scale error. To the authors’ best knowledge, this is the first reported portable NMR flowmeter.