A Highly Integrated Readout Circuit Applied To Nuclear Magnetic Resonance Signals

Nuclear magnetic resonance(NMR)technology is widely used in the field of biomedicine due to its non-destructive testing characteristics.Currently,the most advanced NMR analyzer is based on superconducting magnets as the main magnetic field source,which can achieve extremely high resolution.However,it will also bring the disadvantages of bulky equipment,expensive architecture,and complicated operations.After entering the 21st century,more and more instruments are developing towards portability and intelligence.Therefore,this paper aims to study a kind of NMR readout circuit with high integration in small magnet and low field.This paper designs a readout system for NMR signal analysis.The system architecture is mainly divided into a transmitting circuit used to excite resonance and a receiving circuit used to receive resonance signals.In the transmitting circuit,a duty cycle modulated ring oscillator is used to generate the Larmor frequency pulse in the target main magnetic field.And the pulse signal is amplified by a Class-D power amplifier,so that it has the ability to excite resonance.Traditional receiver generally does not contain an analog-to-digital converter.In order to achieve a high degree of integration,the quantization part of the ADC that should be cascaded off-chip at the back of the receiver is completed in the same on-chip system,which can output real-time quasi-digital time-domain pulse width signal.The receiving circuit includes a front-end low-noise amplifier,a down-conversion module for mixing and filtering,and a time-domain quantization module for the ADC.Among them,the front-end LNA adopts the mutual-existence mode of two-stage cascade and inter-stage matching to ensure sufficient gain while improving noise and linearity.In addition,this paper proposes a current-mode mixer to reduce the requirements of power supply voltage and local oscillator signal amplitude.At the same time,a G_m-C active low-pass filter is designed to compensate the gain of the entire RF front-end.The time-domain quantizer based on voltage-phase conversion is adopted,which has excellent linearity and low noise performance,while reducing the influence of the power consumption and process deviation.The NMR readout circuit designed in this paper is based on the 0.18μm CMOS process of Global Foundries.The preliminary electrical characteristics test work has been completed through tape-out.Under 1.2V power supply voltage,the transmitting circuit can generate 24MHz pulse signal stably.After PA amplification,the voltage peak-to-peak value is 3.2V,the output power is about 64m W,and there are 7-level of different adjustable duty cycle.The input reference noise of the LNA is only 1.4n V/√Hz at the 24MHz working frequency point which shows excellent noise performance.Besides,the input 1d B compression point and IIP3 are-9.31d Bm and-5.98d Bm respectively,showing good linearity.For the quantization part of the ADC,the INL reaches the maximum value of the 0.01LSB when the input offset voltage is 15m V.The core area of the transmitting circuit is only 357.20μm×97.70μm,and the receiving circuit is 410.40μm×420.50μm.Moreover,the power consumption of the receiving part is just 0.58m W.The final test shows that there is no big difference between the performance indicators and the simulation results,which meets the system design requirements.