Paper presentation Ultra-Portable Devices Paper: Lourans Samid, - - PowerPoint PPT Presentation

Paper presentation – Ultra-Portable Devices

Paper: Presented by:

Lourans Samid, Yiannos Manoli, A Low Power and Low Voltage Continuous Time ∑Δ Modulator, ISCAS, pp 4066 - 4069, 23 – 26 May, 2005.

Dejan Radjen

2009-11-17 1 Paper Presentation - Ultra Portable Devices

Outline

• Introduction to Δ∑ modulators
• Continuous Time Δ∑ modulators
• Amplifier Requirements
• Measurement Results
• Summary and Conclusions

2009-11-17 2 Paper Presentation - Ultra Portable Devices

Introduction to Δ∑ modulators

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Introduction to Δ∑ modulators

• White noise approximation is accurate if the input signal is

changing rapidly and is random

• White noise approximation is also more accurate as the number of

bits N in the quantizer increases

• For a one bit quantizer (a comparator) the white noise

approximation is least accurate – but it is used anyway with proper watchfulness

• Maximum theoretical SNR for an ideal ADC using white noise

approximation: SNRdB = 6.02N + 1.76

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Introduction to Δ∑ modulators

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General Δ∑ modulator

Linear model Using the linear model the output can be written as: Y(z) = STF(z)X(z) + NTF(z)E(z) STF(z) = Signal Transfer Function NTF(z) = Noise Transfer Function

Introduction to Δ∑ modulators

• Implementation example of a 2:nd order 1 bit Δ∑-modulator

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STF = z-1 NTF = (1 – z-1)2 Second order noise shaping

Continuous Time Δ∑ Modulators

• Continuous Δ∑ modulators offer implicit anti-aliasing filter

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Modulator Topology

Distributed feedback nth order CT- loop filter

Continuous Time Δ∑ - Modulators

• Excess loop delay τd is the delay between the quantizer output signal

and the DAC output signal

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Excess Loop Delay

Continuous Time Δ∑-modulators

Coefficient Mismatch

• In DT-Δ∑ modulators the coefficients are given by ratios between

capacitors

• In CT- Δ∑ modulators the coefficients are decided by RC – time

constants with tolerances of ± 30 %

• An additional tuning circuit can be added to improve the accuracy
• f the RC constants
• Another method is to use a less aggressive noise transfer function

by moving the poles

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Continuous Time Δ∑ - Modulators

• a. Without moving the poles
• b. With moving the poles

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Coefficient Mismatch

Continuous Time Δ∑-modulators

Clock Jitter

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Error signal Exponential DAC output signals reduce sensitivity to clock jitter

Continuous Time Δ∑-modulators

Integrator Implementation

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Active RC-integrator GmC - integrator

Amplifier Requirements

• Finite DC-gain of the amplifiers cause quantization noise leakage.

A rule of thumb is Adc > OSR

• Finite Gain bandwidth affects the noise transfer function and can

cause instability

• Distortion, Slew rate and noise limitations are primarily due to the

amplifier in the first integrator

• Most power has to be spent on the amplifier in the first integrator

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Measurement Results

Measurement Results for the Third Order Continuous Time Δ∑ modulator

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Measurement Results

Measurement Results for the Third Order Continuous Time Δ∑ modulator

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Summary and Conclusions

• The paper mainly discusses different non idealities present in CT

Δ∑ modulators

• Continuous time Δ∑ modulators are more suitable for low power

design than their discrete time counterparts

• The price paid is increased sensitivity to component mismatch,

excess loop delay and clock jitter

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