An Active CMOS NMR Field Probe with Custom Transceiver and ΣΔ Modulator ASICs and an Optical Link

In this paper, we present a CMOS-based miniaturized magnetic resonance (MR) system for magnetic field and trajectory mapping incorporating an electromagnetic interference (EMI) robust optical link. The system consists of a custom-designed, CMOS-based active NMR field probe, a custom-designed, single-bit ΣΔ modulator and a commercial optical link. The low-IF architecture of the transceiver ASIC, reduces the required bandwidth in the digitizer to a few hundred kilohertz, enabling the use of a single-bit ΣΔ modulator, whose high frequency, one bit data stream is ideally suited for feeding the following optical link. The custom ΣΔ modulator uses a third order loop filter and employs a 12-tap FIR DAC to increase its robustness against clock jitter. It is manufactured in a 0.18 μm CMOS technology and achieves an SNDR of 72.19 dB over a Nyquist bandwidth of 800 kHz. In the proposed setup, the 1-bit output stream of the ΣΔ modulator directly feeds the driver of a commercial optical fiber link to transmit the NMR data from inside the NMR magnet to the remotely located digital signal processing unit with a very high robustness against EMI. In-situ NMR experiments with a 1.45 T benchtop magnet using a silicone sample demonstrate the very good performance of the overall system, achieving a frequency resolution of 12.6 Hz or 200 ppb in the estimation of the sample's NMR resonance frequency. This corresponds to an effective magnetic field resolution of 290 nT.