Automatic Model Refinement of GmC Integrators for High-Level Simulations of Continuous-Time...

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Automatic Model Refinement of GmC Integrators for High-Level Simulations

of Continuous-Time Sigma-Delta Modulators

Michel VasilevskiHassan Aboushady, Marie-Minerve Louërat

Laboratory LIP6 University Pierre and Marie Curie, Paris 6, France

May 2009

M. Vasilevski Laboratory LIP6, University Paris6 2

1.Motivations

2.GmC Model Refinement

3.Characterization Flow

4.Results and Application

5.Conclusion

Outline

1.Motivations

5.Conclusion

Outline

Motivations : Analog Design FlowSystem-LevelSystem-LevelSpecificationsSpecifications

Modify Modify ParametersParameters

SimulationSimulation

PerformancePerformanceanalysisanalysis

OKOKCircuitCircuit

TopologyTopologyCircuit-LevelCircuit-Level

SpecificationsSpecifications

Circuit OKCircuit OK

Sized NetlistSized Netlist

Sizing : Manual Sizing : Manual

Model Model RefinementRefinement

+TsAint

With Accurate Integrators ModelWith Ideal Integrators Model

Continuous-Time Modulator

TsAint

)...2p/s1)(1p/s1()...2z/s1)(1z/s1(

A)s(H int

1.Motivations

5.Conclusion

Outline

GmC Model Refinement

ii iioi oi

)s(H int

GmC idealTransfer Function:

GmC Integrator Design Results

I0 7.6 μA

Desired fT 1.36 MHz

Simulated fT 1.29 MHz

C 3.6 pF

ΣΔ Specifications

BW 200 kHz

OSR 64

Aint (Integrator Gain) 1/3

AΣΔ (Input Amplitude) -5dB

GmC Non-Idealities

CMOS Process:0.13 μm

Simplified GmC Model

DDV0I 0I 0I 0I 0I 0I

ii iioi oi

1p/s11z/s1

Simplified models are not sufficiently accurate

[Zele,Allstot,JSSC’96][Zele,Allstot,JSSC’96]

DDV0I 0I 0I 0I 0I 0I

ii iioi oi

Accurate Cascoded GmC Model

)2p/s1)(1p/s1()2z/s1)(1z/s1(

1.Motivations

5.Conclusion

Outline

Characterization Flow : Conventional methodSystem-Level System-Level

Specifications : SNR, BWSpecifications : SNR, BW

Modify Modify Aint, OSRAint, OSR

SNR analysisSNR analysisscalingscaling

OKOKGmCGmC

TopologyTopologyGmC SpecificationsGmC Specifications

Sizing : Manual Sizing : Manual

Circuit OKCircuit OK

GmC ModelGmC Model

Zeros/PolesZeros/PolesExtractionExtraction

)2p/s1)(1p/s1()2z/s1)(1z/s1(

TsAint

SystemC-AMS C++ based tool. Fixed step discrete-time System-Level simulator. Adapted to mixed analog-digital systems modeling. Ongoing standardization (extension SystemC).

CAIRO+: C++ based tool. Exact Bsim3v3 models are used for transistor sizing. Small-Signal parameters extraction. Suited for technology migration.

Full interoperability.

Characterization Flow : Full Automation

System-Level System-Level Specifications : SNR, BWSpecifications : SNR, BW

Modify Modify Aint, OSRAint, OSR

SNR analysisSNR analysisscalingscaling

GmCGmCTopologyTopology

GmC SpecificationsGmC Specifications

GmC ModelGmC Model

fA sint

Sizing : SynthesisSizing : Synthesis

Small-Signal ParametersSmall-Signal Parameters

Symbolic Symbolic expressionexpression

Zeros/PolesZeros/Poles

Characterization Flow : Proposed Method

)2p/s1)(1p/s1()2z/s1)(1z/s1(

Characterization Flow : Full Automation

Conventional method Proposed methodDifficult Interoperability System/Circuit Level: Manual interventions for transfer function characterization.

Fully compatible system to circuit level interface: C++ based tools, SystemC-AMS/CAIRO+.

Simulation-Aided transistor sizing: Time consuming.

« Knowledge » based sizing: Using CAIRO+ for accurate transistor sizing, suited for technology migration.

1.Motivations

5.Conclusion

Outline

2nd order CT in a 0.13 m CMOS

Specifications: SNR= 60 dB, OSR = 64, BW = 200 kHz

output Power Spectral DensityGmC Frequency Response

SNR=47dBSNR=47dB

SNR=68dBSNR=68dB

Transistors length: L1=10μm, L3=9 μm

SNR=68dBSNR=68dB

2nd order CT in a 0.13 mm CMOS

Specifications: SNR= 60 dB, OSR = 64, BW = 200 kHz

output Power Spectral DensityGmC Frequency Response

Transistors length: L1=3 μm, L3=0.18 μm

Technology migration : 0.25 μm / 0.13 μm

0.25 μm 0.13 μm

BW=200kHz 68.2dB 68.4dB

BW=10MHz 52.1dB 67.3dB

Specifications: SNR= 60 dB, OSR = 64, BW = 200 kHz/10MHz

SNR=52dBSNR=52dB

SNR~68dBSNR~68dB

5 - Conclusion

• Automatic refinement of high-level system models based on:

• Exact symbolic expressions for small signal analysis

• Accurate BSIM3v3 transistor models

• Homogeneous Environment (C++):

• High-Level Simulation => SystemC-AMS

• Circuit synthesis and characterization => CAIRO+

• Proposed method illustrated on the GmC integrator of a 2nd order modulator.

Automatic Model Refinement of GmC Integrators for High-Level Simulations of Continuous-Time...

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