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Lect2UP360_(100329)

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

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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
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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 0 f f B -f B 0 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 0 f S 2 f S f Antialiasing Filter f S 2 f B -f B 0 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
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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 Decimal Binary Thermometer Gray Two’s Complement 0 000 0000000 000 000 1 001 0000001 001 111 2 010 0000011 011 110 3 011 0000111 010 101 4 100 0001111 110 100 5 101 0011111 111 011 6 110 0111111 101 010 7
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