Lect2UP390_(100329) - Lecture 390 Oversampling ADCs Part I...

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Lecture 390 – Oversampling ADCs – Part I (3/29/10) Page 390-1 CMOS Analog Circuit Design © P.E. Allen - 2010 LECTURE 390 – OVERSAMPLING ADCS – PART I LECTURE ORGANIZATION Outline • Introduction • Delta-sigma modulators • Summary CMOS Analog Circuit Design, 2 nd Edition Reference Pages 698-705 Lecture 390 – Oversampling ADCs – Part I (3/29/10) Page 390-2 CMOS Analog Circuit Design © P.E. Allen - 2010 INTRODUCTION What is an oversampling converter? An oversampling converter uses a noise-shaping modulator to reduce the in-band quantization noise to achieve a high degree of resolution. • What is the range of oversampling? The oversampling ratio, called M , is a ratio of the clock frequency to the Nyquist frequency of the input signal. This oversampling ratio can vary from 8 to 256. - The resolution of the oversampled converter is proportional to the oversampled ratio. - The bandwidth of the input signal is inversely proportional to the oversampled ratio. • What are the advantages of oversampling converters? Very compatible with VLSI technology because most of the converter is digital High resolution Single-bit quantizers use a one-bit DAC which has no INL or DNL errors Provide an excellent means of trading precision for speed (16-18 bits at 50ksps to 8-10 bits at sampling rates of 5-10Msps). • What are the disadvantages of oversampling converters? Difficult to model and simulate Limited in bandwidth to the clock frequency divided by the oversampling ratio
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Lecture 390 – Oversampling ADCs – Part I (3/29/10) Page 390-3 CMOS Analog Circuit Design © P.E. Allen - 2010 Nyquist Versus Oversampled ADCs Conventional Nyquist ADC Block Diagram: Fig.10.9-01 Digital Processor y ( kT N ) x ( t ) Filtering Sampling Quantization Digital Coding Oversampled ADC Block Diagram: Fig.10.9-02 Decimation Filter y ( kT N ) x ( t ) Filtering Sampling Quantization Digital Coding Modulator Components: Filter - Prevents possible aliasing of the following sampling step. Sampling - Necessary for any analog-to-digital conversion. Quantization - Decides the nearest analog voltage to the sampled voltage (determines the resolution). Digital Coding - Converts the quantizer information into a digital output signal. Lecture 390 – Oversampling ADCs – Part I (3/29/10) Page 390-4 CMOS Analog Circuit Design © P.E. Allen - 2010 Frequency Spectrum of Nyquist and Oversampled Converters Definitions: f B = analog signal bandwidth f N = Nyquist frequency (two times f B ) f S = sampling or clock frequency M = f S f N = f S 2 f B = oversampling ratio Frequency prespective: Fig.10.9-03 0.5 f N = 0.5 f S f B f S = f N 0 0 Amplitude f f B = 0.5 f N 0.5 f S f S = Mf N 0 0 f f N Anti-aliasing filter Anti-aliasing filter Signal Bandwidth Signal Bandwidth Transition band Transition band Conventional ADC with f B 0.5 f N =0.5 f S . Oversampled ADC with f B 0.5 f N << f S .
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Lecture 390 – Oversampling ADCs – Part I (3/29/10) Page 390-5 CMOS Analog Circuit Design © P.E. Allen - 2010 Quantization Noise of a Conventional (Nyquist) ADC Multilevel Quantizer: The quantized signal y can be represented as, y = Gx + e where G = gain of the ADC, normally 1 e
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Lect2UP390_(100329) - Lecture 390 Oversampling ADCs Part I...

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