Research And Design Of High-Performance Multi-mode Ultrasound AFE

Ultrasonic imaging technology is widely used in the field of medical diagnosis.With the increasing demand of portable equipment for clinical diagnosis,the integrated ultrasonic signal processing circuit with higher performance has gradually become a research hotspot.The analog front end(AFE),as the first stage of processing ultrasonic transducer output signal in the integrated imaging system,has a significant impact on the performance of the whole system.The high frequency ultrasonic imaging simulation front-end designed in this thesis consists of eight channels,each of which is composed of four modules: low noise amplifier(LNA),voltage-controlled attenuator(VCAT),programmable gain amplifier(PGA)and antialiasing filter(AAF).This thesis analyzes the target performance parameters of each module in detail according to the signal characteristics of the transducer output,and studies the theory of equivalent noise,noise elimination,S-parameters of dual port network and necessary knowledge of impedance matching for system design.The LNA designed in this thesis can adjust the input impedance according to the characteristics of the output matching layer of different ultrasonic transducers.It can match to four typical internal resistance values of transducers to achieve the maximum power transmission efficiency of weak ultrasonic signals.At the same time,the power gain of the LNA is adjustable.Under different working modes,the noise figure is between 0.99 d B and 2.5d B.The parameters of S11 and S22 of the LNA are less than-10 d B.Under different working modes,S21,the gain parameter,is between 29 d B and 33 d B.The power consumption of the LNA is 23.4m W at the working voltage of 1.8V.When LNA input impedance is matched to 50 Ω,P1 d B is-13.07 dbm.The attenuator is composed of seven attenuating units,controlled by external voltage to achieve the attenuator gain range of 0 to-42 d B by-6d B.The attenuator’s attenuating network is composed of MOS transistors working in the deep linear region as the resistance,and high linearity is ensured by fully differential structure;the programmable gain amplifier adjusts the gain by changing the closed-loop resistance feedback network of the operational amplifier.The operational amplifier adopts the gain bootstrapping structure,the unit gain bandwidth product is 300 MHz,and the phase margin is 84 degrees.The PGA can achieve 18 db,24d B and 30 d B gains.The anti-aliasing filter is a fourth-order Butterworth low-pass filter,which is used to prevent aliasing during sampling of the ADC.Its roll off coefficient is-80 d B/Decade.the cut-off frequency can be adjusted between 20 MHz,30MHz and 40 MHz by adjusting the capacitance of the feedback capacitor array.There’s no ripple in Butterworth filter passband,so the signal is not easy to be distorted in the process of transmission in the channel.The design of this thesis is based on tsmc18 rf technology.Each module of eight-channel ultrasonic AFE is directly coupled.The overall gain range is [3d B,60 d B],the gain step is 6d B,the noise figure is 1.36 d B when the gain is the highest,and the power consumption is 36 m W/channel.Comprehensive simulation performance analysis and layout design of the whole analog front-end circuit were finished in this work.The PCB level test bench was designed to test the main functions of the AFE chip.