Analysis Of Substrate Noise Coupling In Analog-digital Converter

With the development of semiconductor technology, the mixed-signal systems areused more widely, but substrate coupling noise has become a non-ignorable problem. Inthe system of analog to digital converter, there are number of digital circuits near theanalog module. The noise current caused by the digital circuits injects into the substratethrough various mechanisms and propagates to the sensitive module through substrate,it can effect on circuits’ performance, even the entire system.Currently, the mainstream emulator does not provide a substrate model, so acomplete and effective substrate model is necessary to analyze substrate noise inadvance to ensure the accuracy of the circuits after the flow sheet.The paper analyzes the injection, propagation and influence on sensitive circuits ofthe digital noise, and introduces the methods how to protect from the noise at thesethree aspects. The3D hybrid model which tradeoffs between complexity and accuracyand combines the3D RC model and macro resistance model is established about lightdoped substrate of the ADC, and its effectiveness is verified by TCAD. At the same time,the paper analyzes the coupling noise which comes from power/ground and establishesthe coupling path model. The model is applied to the Delta-Sigma modulator circuitsand a SPECTRE simulation is carried out, the signal-to-noise ratio (SNR) of the circuitreduces by25dB compared to the ideal case because of the substrate noise.This paper carries on two kinds isolation structures of substrate noise aimed atSMIC18process. The one is a guard ring, if the width is constant, it has the bestisolation effectiveness when the distance from the analog circuits is13.33um. If thedistance from the analog circuits is20um and remains constant, the isolationeffectiveness of a guard ring increases as the width of guard ring increases. The guardrings at differents situations are applied to the first level switched capacitor integrator(SCI) of the Delta-Sigma modulator for qualitative analysis. A guard ring whose widthis20um, the distance from analog circuits is13.33um is applied to the Delta-Sigmamodulator and the SNR is raised by20dB than without the guard ring. The other is thedeep N_well, this paper analyzes the relationship between isolation effectiveness andthe regions of deep N_well, and applies each case to the first level SCI. The deepN_well in the substrate of the first level SCI is applied to the Delta-Sigma modulatorcircuit and the SNR of this circuit is raised by9.3dB than without the deep N_well.