GT ECE 6414 - OVERSAMPLING CONVERTERS

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CMOS Analog IC Design - Chapter 10 Page 10.9-1 Chapter 10 - DA and AD Converters (6/4/01) © P.E. Allen, 2001 10.9 - OVERSAMPLING CONVERTERS 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 possible performance of an oversampled converter? The performance can range from 16 to 18 bits of resolution at bandwidths up to 50kHz to 8 to 10 bits of resolution at bandwidths up to 5-10MHz. 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 signal to be converted 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 What are the disadvantages of oversampling converters? Difficult to model and simulate Limited in bandwidth to the clock frequency divided by the oversampling ratioCMOS Analog IC Design - Chapter 10 Page 10.9-2 Chapter 10 - DA and AD Converters (6/4/01) © P.E. Allen, 2001 NYQUIST VERSUS OVERSAMPLED ADCs Conventional Nyquist ADC Block Diagram: Oversampled ADC Block Diagram: 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 signal.CMOS Analog IC Design - Chapter 10 Page 10.9-3 Chapter 10 - DA and AD Converters (6/4/01) © P.E. Allen, 2001 FREQUENCY SPECTRUM OF NYQUIST AND OVERSAMPLED CONVERTERS Definitions: fB = analog signal bandwidth fN = Nyquist frequency (two times fB) fS = sampling or clock frequency M = fSfN = fS2fB = oversampling ratio Frequency spectrums:CMOS Analog IC Design - Chapter 10 Page 10.9-4 Chapter 10 - DA and AD Converters (6/4/01) © P.E. Allen, 2001 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 = quantization error The mean square value of the quantization error isCMOS Analog IC Design - Chapter 10 Page 10.9-5 Chapter 10 - DA and AD Converters (6/4/01) © P.E. Allen, 2001 e2rms = SQ = 1∆ ⌡⌠-∆/2∆/2 e(x)2dx = ∆212CMOS Analog IC Design - Chapter 10 Page 10.9-6 Chapter 10 - DA and AD Converters (6/4/01) © P.E. Allen, 2001 QUANTIZATION NOISE OF A CONVENTIONAL (NYQUIST) ADC - CONTINUED Spectral density of the sampled noise: When a quantized signal is sampled at fS (= 1/τ), then all of its noise power folds into the frequency band from 0 to 0.5fS. Assuming that the noise power is white, the spectral density of the sampled noise is, E(f) = erms2fS = erms2τ where τ = 1/fS and fS = sampling frequency The inband noise energy no is no2 = ⌡⌠0fB E2(f)df = e2rms (2fBτ) = e2rms 2fBfS = e2rms M no = ermsM What does all this mean? • One way to increase the resolution of an ADC is to make the bandwidth of the signal, fB, less than the clock frequency, fS. In otherwords, give up bandwidth for precision. • However, it is seen from the above that a doubling of the oversampling ratio M, only gives a decrease of the inband noise, no, of 1/ 2 which corresponds to -3dB decrease or an increase of resolution of 0.5 bits The conclusion is that reduction of the oversampling ratio is not a very good method of increasing the resolution of a Nyquist analog-digital converter.CMOS Analog IC Design - Chapter 10 Page 10.9-7 Chapter 10 - DA and AD Converters (6/4/01) © P.E. Allen, 2001 OVERSAMPLED ANALOG-DIGITAL CONVERTERS Classification of oversampled ADCs: 1.) Straight-oversampling - The quantization noise is assumed to be equally distributed over the entire frequency range of dc to 0.5fS. This type of converter is represented by the Nyquist ADC. 2.) Predictive oversampling - Uses noise shaping plus oversampling to reduce the inband noise to a much greater extent than the straight-oversampling ADC. Both the signal and noise quantization spectrums are shaped. 3.) Noise-shaping oversampling - Similar to the predictive oversampling except that only the noise quantization spectrum is shaped while the signal spectrum is preserved. The noise-shaping oversampling ADCs are also known as delta-sigma ADCs. We will only consider the delta-sigma type oversampling ADCs.CMOS Analog IC Design - Chapter 10 Page 10.9-8 Chapter 10 - DA and AD Converters (6/4/01) © P.E. Allen, 2001 OVERSAMPLING ANALOG-DIGITAL CONVERTERS - CONTINUED General block diagram of an oversampled ADC: Components of the Oversampled ADC: 1.) ∆Σ Modulator - Also called the noise shaper because it can shape the quantization noise and push the majority of the inband noise to higher frequencies. If modulates the analog input signal to a simple digital code, normally a one-bit serial stream using a sampling rate much higher than the Nyquist rate. 2.) Decimator - Also called the down-sampler because it down samples the high frequency modulator output into a low frequency output and does some pre-filtering on the quantization noise. 3.) Digital Lowpass Filter - Used to remove the high frequency quantization noise and to preserve the input signal. Note: Only the modulator is analog, the rest of the circuitry is digital.CMOS Analog IC Design - Chapter 10 Page 10.9-9 Chapter 10 - DA and AD Converters (6/4/01) © P.E. Allen, 2001 FIRST-ORDER, DELTA-SIGMA MODULATOR Block diagram of a first-order, delta-sigma modulator: Components: • Integrator (continuous or discrete time) • Coarse quantizer (typically two levels) - A/D which is a comparator for two levels - D/A which is a switch for two levels First-order


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GT ECE 6414 - OVERSAMPLING CONVERTERS

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