Research And Design Of Key Modules Of Ultrasound Imaging Analog Front End

Rapid development of people's life rhythm requires that the treatment and testing of the body can be done quickly and accurately.The ultrasound imaging system plays an important role in the modern medical system.Ultrasound attenuated by human tissue and bones,the signal received by the transducer is weak.The quality of this small signal processing performed by the analog front end determines the imaging quality.Ultrasound imaging systems in China focus on low-end products.Its high-end analog chips are heavily dependent on imports,so the research and design of the ultrasound analog front-end chip with independent property rights is extremely important.The basic modules of the abdominal ultrasonic analog front end are:low noise amplifier,programmable gain amplifier and low-pass filter.In this article low noise amplifier uses noise cancellation technology,using the signal and noise phase reversal features to eliminate noise and increase gain.The input impedance of the low noise amplifier is matched to 50?.The input and output matching parameters S₁₁ and S₂₂ after simulation and optimization are all less than-10dB,the gain parameter S₂₁ is 16.1dB,the reverse gain S₁₂ is-45dB,the noise figure is 1.9dB and the third-order intercept point is-1.89 dBm.The circuit input is matched well,the gain meets the requirements of the input stage,the input and output have good isolation,the noise is small and the linearity is high;The programmable gain amplifier consists of two stages circuits:coarse and fine-tuned programmable gain amplifiers.Coarse PGA couples two degenerate resistor fixed-gain amplifiers with a feedback loop in cascade.The fine-tuning PGA uses a closed-loop resistive feedback structure to achieve fine-tuning of the gain by changing the ratio of the resistors.Adding a buffer at the input isolates the effect of the output resistance of the previous stage on the input resistance of this stage.The coarse tuning stage achieves a0⁵6dB gain range with a step size of 14dB.A gain of 14dB,the third-order intercept point is 2.98dBm and the noise figure is 24.5dB.A gain of 56dB,the third-order intercept point is 2.95dBm and the noise figure is 26.5dB.The fine-tuning stage achieves a gain range of0¹4dB with a steps of 2dB and the third-order intercept point is 12.2dBm.The first stage of the op amp is a folded cascode structure and the output stage is a class-AB structure.The gain bandwidth product is 200MHz.The phase margin is 73 degrees.The simulation results show that adding a negative feedback loop can make the circuit noise figure smaller,while the linearity is higher and the closed-loop resistance feedback structure has better linearity.The low-pass filter uses two stages of double second-order multiple feedback Butterworth.The main function of the structure is to prevent ADC aliasing.The designed LPF cutoff frequency is 2MHz,the stopband attenuation is-75dB and the group delay is2.9×10^-7s.The flatness of the filter is high and the group delay is small to meet the attenuation requirements.The bandgap reference voltage source adopts V_be linearization technology.The temperature drift coefficient is 7.8ppm/^oC the power supply rejection ratio is-57.39dB and the reference voltage source hardly varies with temperature and small impact by the power supply.These circuits have a good performance.In this paper,TSMC0.18umCMOS technology library is adopted.After the design of each module is completed,they are cascaded together using AC coupling.Overall,a gain range of 16 to 72 dB is achieved with a step size of 2dB.The noise figure is 5.2 dB at the maximum gain and a linearity of-15.9dBm.The cascaded circuit has a small noise figure,high linearity,low nonlinear distortion and can withstand the power of the signal source to meet the design requirements.