Lecture 360 – Characterization of ADCs and Sample and Hold Circuits (3/29/10)

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LECTURE 360 – CHARACTERIZATION OF ADCS AND SAMPLE AND HOLD CIRCUITS LECTURE ORGANIZATION Outline • Introduction to ADCs • Static characterization of ADCs • Dynamic characteristics of ADCs • Sample and hold circuits • Design of a sample and hold • Summary CMOS Analog Circuit Design, 2nd Edition Reference Pages 652-665

CMOS Analog Circuit Design

© P.E. Allen - 2010

Lecture 360 – Characterization of ADCs and Sample and Hold Circuits (3/29/10)

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INTRODUCTION General Block Diagram of an Analog-Digital Converter

x(t) Prefilter

Sample/Hold

Quantizer

Digital Processor

y(kTN)

Encoder

Fig.10.5-1

• Prefilter - Avoids the aliasing of high frequency signals back into the baseband of the ADC • Sample-and-hold - Maintains the input analog signal constant during conversion • Quantizer - Finds the subrange that corresponds to the sampled analog input • Encoder - Encoding of the digital bits corresponding to the subrange

CMOS Analog Circuit Design

© P.E. Allen - 2010

Lecture 360 – Characterization of ADCs and Sample and Hold Circuits (3/29/10)

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Nyquist Frequency Analog-Digital Converters The sampled nature of the ADC places a practical limit on the bandwidth of the input signal. If the sampling frequency is fS, and fB is the bandwidth of the input signal, then fB < 0.5fS Continuous time frequency response of the analog input signal. which is simply the Nyquist relationship which states that to avoid aliasing, the f fS -fB 0 fB sampling frequency must be Sampled data equivalent frequency response where f < 0.5f . B S greater than twice the highest signal frequency. fB fS fS-fB fS 2 Case where fB > 0.5fS causing aliasing. -fB

0

fS fS 2 Use of an antialiasing filter to avoid aliasing. -fB

0

fS+fB 2fS-fB

2fS

2fS+fB

f

f

2fS

Antialiasing Filter -fB

0

fB fS 2

CMOS Analog Circuit Design

f

fS

Fig. 10.5-

© P.E. Allen - 2010

Lecture 360 – Characterization of ADCs and Sample and Hold Circuits (3/29/10)

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Classification of Analog-Digital Converters Analog-digital converters can be classified by the relationship of fB and 0.5fS and by their conversion rate. • Nyquist ADCs - ADCs that have fB as close to 0.5fS as possible. • Oversampling ADCs - ADCs that have fB much less than 0.5fS. Classification of Analog-to-Digital Converter Architectures Conversion Nyquist ADCs Oversampled ADCs Rate Slow Integrating (Serial) Very high resolution