Research On Design Method And Implementation Technology Of Continuous Time Bandpass Sigma-Delta Modulator

In sigma-delta ADC,the quantization noise in A/D conversion process is shaped by an analog modulator based on the oversampling technology,and then the noise is filtered out by a digital filter,so as to achieve A/D conversion with high accuracy and high signal-to-noise ratio.The structure of the analog modulator determines the accuracy and speed of the entire converter.Continuous-time band-pass Sigma-Delta modulator can effectively improve the accuracy of A/D conversion and reduce system power consumption due to its inherent anti-aliasing effect and band-pass shaping characteristics.It conforms to the development trend of low power consumption and high precision in the field of digital communication and portable electronic equipment in recent years.Research on its design method has great significance in digital information age nowadays.Considering the comprehensive consideration of speed,power consumption and accuracy,this paper designs a 3-order 1-bit continuous-time complex band-pass modulator that can be applied to low-IF receivers.The overall structure is formed by cross coupling of two orthogonal paths I and Q,the single channel is a single-loop feedforward structure continuous-time low-pass modulator,which is composed of a fully differential active RC integrator,dynamic comparator,sampling switch,and resistive FIR feedback DAC.This design introduces FIR filtering in front of the SCR feedback DAC,and obtains a feedback waveform similar to the multi-bit DAC in the unit quantization structure,combining the good linearity of the unit quantization structure and the low clock jitter sensitivity of the multi-bit quantization structure.In this paper,the Delta-Sigma Toolboox is used to obtain the corresponding noise transfer function of the discrete-time low-pass modulator.The impulse response invariant transform is used to implement the DT-CT transform to obtain the transfer function of the continuous-time low-pass modulator.Based on the transfer function,the single-channel and the overall Simulink model of the modulator are established in MATLAB to verify the overall performance of the system and the influence of non-ideal factors such as the finite gain bandwidth product of the op amp,clock delay,and clock jitter on the modulator performance are explored.The transistor-level circuit design,simulation verification and layout drawing of the modulator are completed on Cadence platform.In this paper,the circuit design is implemented in CSMC0.18μm process and 1.6V operatingvoltage.When the input amplitude is sine and cosine signal with amplitude of 1V and frequency of 300 KHz and the oversampling rate is 60,the SNR of the modulator reaches 83.1dB,and the dynamic range reaches 84 dB.The overall power consumption of the circuit is 3.7mW,the layout area is 2.1mm×2.2mm,and the overall performance meets the design requirements.