Design And Research Of Filter Circuit In Ultrasound Receive Path

In order to make medical diagnosis more accurate,the quality of the imaging of tissues and organs is of great importance.Ultrasound imaging is a technique that uses ultrasonic waves as a carrier to detect the inside of a living organism and extracts effective signals in echoes through an ultrasonic system composed of a transducer,an analog receiving circuit,and a digital processing unit,and performs imaging.Ultrasound imaging is widely used in the field of medical imaging due to its advantages such as low risk,low cost,and portability.Among them,the performance of the analog receiving circuit decides the signal-to-noise ratio of the effective signal in the echo,which needs to be carefully designed and optimized.This paper systematically studies and designs the filter module in the ultrasonic receiver circuit,including a programmable lowpass filter and a narrowband bandpass filter.The design tool for this article is Cadence.The design environment is a TSMC 0.18?m standard process library with 1.8V supply voltage.In the normal mode,the ultrasound system can perform fetal imaging and other operations.In this case,a programmable lowpass filter needs to be used in the receiver channel for anti-aliasing processing.The 3dB cutoff frequencies are 3,6,9,12,15,and 18 MHz,respectively.This paper designs a 6th order Active-RC Chebyshev lowpass filter based on design requirements.In order to facilitate the adjustment of the cut-off frequency,a cascade filter synthesis method was adopted and a Tow-Thomas II type Biquad unit was selected.A two-stage opamp with Miller compensation is designed as the core unit in the filter.A dual differential pair common-mode feedback network is used to stabilize the output common-mode level of the opamp.The opamp has a DC gain of 81.11dB,a unity gain bandwidth of 150.2MHz,a phase margin of 50.16o,a slew rate of 47.58V/?s,and an equivalent input noise of 14.25nV/sqrt(Hz).Aiming at the deterioration of filter performance caused by the limited gain and bandwidth products of the opamp,three different solutions were compared and analyzed,including improving the opamp,compensation resistor and compensation capacitor,and choosing a compensation capacitor method with good practicality.A bandgap reference circuit is designed to provide a stable reference voltage for the ultrasonic receiver circuit.The main part includes the startup circuit,error amplifier,and transistor components.Among them,the error amplifier uses a folded cascode structure to meet the needs of gain and swing.The temperature drift coefficient of the bandgap reference is 9.3 ppm/°C and the DC current consumed is 22.4?A.Simple and efficient programmable digital logic is designed.Transfer gate switches are used to avoid threshold voltage losses and introduce smaller on-resistance.An exponentially increasing capacitance array is used to reduce the number of control signals.Finally,the-3dB cutoff frequency of the filter is basically in accordance with the design requirements.The attenuation at the 2x cutoff frequency point exceeds 45dB,the maximum ripple of the passband is 1.4dB,the IIP3 is 24.42dBm,the maximum group passband delay is 104.5ns,the equivalent input noise is 98.29nV/sqrt(Hz),and the consumption of quiescent current is 3.49mA,which meets the design requirements in normal mode.In continuous acoustic Doppler mode,the ultrasound system can perform blood flow detection and other operations.At this time,a narrowband bandpass filter is used in the receiving channel to obtain a useful signal near 20 MHz,and its-3dB bandwidth is 4 to 6MHz.Considering power consumption,in-band flatness and design complexity,an eighth-order Gm-C Butterworth band-pass filter is designed in this paper,and the filter is integrated by the signal flow graph method.Nauta-OTA is used to compensate for the lack of linearity of the Gm-C filter.The non-ideal effects of noise,the finite output impedance of the integrator,and the internal node dynamic range were analyzed and optimized.The simulation results show that the filter's-3dB bandwidth is 4.88MHz,the passband gain is0.74dB,the neighbor signal attenuation is(-29dB@15MHz,-20.77dB@25MHz),the linearity is 9.64dBm,the maximum group delay is 262.66ns,the passband ripple is less than 0.4dB,the equivalent input noise is 22.04nV/sqrt(Hz),and the quiescent current consumption is 0.418mA,which is consistent with the design requirements of the continuous acoustic Doppler mode.The relevant design details of the layout of the integrated circuit are introduced,and the layout design of the lowpass filter and the bandpass filter is completed using Virtuoso tools.The area of the lowpass filter is 1.1*0.5mm~2,and the area of the bandpass filter is0.28*0.13mm~2.