Lect2UP360_(100329)

Lect2UP360_(100329) - Lecture 360 Characterization of ADCs...

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Unformatted text preview: Lecture 360 Characterization of ADCs and Sample and Hold Circuits (3/29/10) Page 360-1 CMOS Analog Circuit Design P.E. Allen - 2010 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, 2 nd Edition Reference Pages 652-665 Lecture 360 Characterization of ADCs and Sample and Hold Circuits (3/29/10) Page 360-2 CMOS Analog Circuit Design P.E. Allen - 2010 INTRODUCTION General Block Diagram of an Analog-Digital Converter Digital Processor Prefilter Sample/Hold Quantizer Encoder x(t) y(kT N ) 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 Lecture 360 Characterization of ADCs and Sample and Hold Circuits (3/29/10) Page 360-3 CMOS Analog Circuit Design P.E. Allen - 2010 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 f S , and f B is the bandwidth of the input signal, then f B < 0.5 f S which is simply the Nyquist relationship which states that to avoid aliasing, the sampling frequency must be greater than twice the highest signal frequency. f B-f B f f B-f B f S f S-f B f S +f B 2 f S 2 f S-f B 2 f S +f B f-f B f S 2 f S f Antialiasing Filter f S 2 f B-f B f f S 2 f S 2 f S f S Continuous time frequency response of the analog input signal. Sampled data equivalent frequency response where f B < 0.5 f S . Case where f B > 0.5 f S causing aliasing. Use of an antialiasing filter to avoid aliasing. Fig. 10.5- Lecture 360 Characterization of ADCs and Sample and Hold Circuits (3/29/10) Page 360-4 CMOS Analog Circuit Design P.E. Allen - 2010 Classification of Analog-Digital Converters Analog-digital converters can be classified by the relationship of f B and 0.5 f S and by their conversion rate. Nyquist ADCs- ADCs that have f B as close to 0.5 f S as possible. Oversampling ADCs- ADCs that have f B much less than 0.5 f S . Classification of Analog-to-Digital Converter Architectures Conversion Rate Nyquist ADCs Oversampled ADCs Slow Integrating (Serial) Very high resolution <14-16 bits Medium Successive Approximation1-bit Pipeline Algorithmic Moderate resolution <10-12 bits Fast Flash Multiple-bit Pipeline Folding and interpolating Low resolution < 6-8 bits Lecture 360 Characterization of ADCs and Sample and Hold Circuits (3/29/10) Page 360-5 CMOS Analog Circuit Design P.E. Allen - 2010 STATIC CHARACTERIZATION OF ANALOG-TO-DIGITAL CONVERTERS Digital Output Codes Digital Output Codes used for ADCs...
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Lect2UP360_(100329) - Lecture 360 Characterization of ADCs...

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