– 1 – Data Converters Oversampling ADC Professor Y. Chiu - PowerPoint Presentation

Slide1

– 1 –

Data Converters Oversampling ADC Professor Y. ChiuEECT 7327 Fall 2014

Oversampling ADCSlide2

Nyquist-Rate ADC

– 2 –

Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

The “black box” version of the quantization process

Digitizes the input signal up to the Nyquist frequency (fs/2)Minimum sampling frequency (fs) for a given input bandwidthEach sample is digitized to the maximum resolution of the converterSlide3

Anti-Aliasing Filter (AAF)

– 3 –Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

Input signal must be band-limited prior to sampling

Nyquist sampling places stringent requirement on the roll-off characteristic of AAF

Often some oversampling is employed to relax the AAF design (better phase response too)Decimation filter (digital) can be linear-phaseSlide4

Oversampling ADC–

4 –Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

Sample rate is well beyond the signal bandwidth

Coarse quantization is combined with feedback to provide an accurate estimate of the input signal on an “average” sense

Quantization error in the coarse digital output can be removed by the digital decimation filterThe resolution/accuracy of oversampling converters is achieved in a sequence of samples (“average” sense) rather than a single sample; the usual concept of DNL and INL of Nyquist converters are not applicableSlide5

Relaxed AAF Requirement–

5 –Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

Nyquist-rate converters

Oversampling converters

Sub-sampling

Band-pass oversampling

OSR =

f

s

/2

f

mSlide6

Oversampling ADC–

6 –Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

Predictive type

Delta modulation

Noise-shaping typeSigma-delta modulationMulti-level (quantization) sigma-delta modulationMulti-stage (cascaded) sigma-delta modulation (MASH)Slide7

Oversampling–

7 –Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

Nyquist

Oversampled

Sample rate

Noise power

Power

Nyquist

f

s

Δ

2

/12

P

Oversampled

M*

f

s

(

Δ

2

/12)/M

M*P



OSR = MSlide8

Noise Shaping–

8 –Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

Push noise out of signal band

Large gain @ LF, low gain @ HF

→ Integrator?

Slide9

Sigma-Delta (ΣΔ) Modulator

– 9 –

Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

Noise shaping obtained with an integrator

Output subtracted from input to avoid integrator saturation

First-order

ΣΔ

modulatorSlide10

Linearized Discrete-Time Model

– 10 –Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

Caveat: E(z) may be correlated with X(z) – not “white”!Slide11

First-Order Noise Shaping

– 11 –Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

Doubling OSR (M) increases SQNR by 9 dB (1.5 bit/oct)Slide12

SC Implementation–

12 –Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

SC integrator

1-bit ADC

→ simple, ZX detector1-bit feedback DAC → simple, inherently linearSlide13

Second-Order ΣΔ Modulator

– 13 –

Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

Doubling OSR (M) increases SQNR by 15 dB (2.5 bit/oct)Slide14

2nd-Order ΣΔ Modulator (1-Bit

Quantizer)–

14

–

Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014Simple, stable, highly-linearInsensitive to component mismatchLess correlation b/t E(z) and X(z)Slide15

Generalization (Lth-Order Noise Shaping)

– 15 –

Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

Doubling OSR (M) increases SQNR by (6L+3) dB, or (L+0.5) bit

Potential instability for

3rd- and higher-order single-loop

ΣΔ

modulatorsSlide16

ΣΔ vs. Nyquist

ADC’s– 16

–

Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

ΣΔ ADC output (1-bit)Nyquist ADC output

ΣΔ

ADC behaves quite differently from Nyquist converters

Digital codes only display an “average” impression of the input

INL, DNL, monotonicity, missing code, etc. do not directly apply in

ΣΔ

converters

→ use SNR, SNDR, SFDR insteadSlide17

Tones–

17 –Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

The output spectrum corresponding to V

i

= 0 results in a tone at

f

s

/2, and will get eliminated by the decimation filter

The 2nd output not only has a tone at

f

s

/2, but also a low-frequency tone –

f

s

/2000 – that cannot be eliminated by the decimation filterSlide18

Tones–

18 –Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

Origin – the quantization error spectrum of the low-resolution ADC (1-bit in the previous example) in a

ΣΔ

modulator is NOT white, but correlated with the input signal, especially for idle (DC) inputs. (R. Gray, “Spectral analysis of sigma-delta quantization noise”)Approaches to “whitening” the error spectrumDither – high-frequency noise added in the loop to randomize the quantization error. Drawback is that large dither consumes the input dynamic range.

Multi-level quantization. Needs linear multi-level DAC.

High-order single-loop

ΣΔ

modulator. Potentially unstable.

Cascaded (MASH)

ΣΔ

modulator. Sensitive to mismatch.Slide19

Cascaded (MASH) ΣΔ Modulator

– 19 –

Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

Idea: to further quantize E(z) and later subtract out in digital domain

The 2nd quantizer can be a

ΣΔ

modulator as wellSlide20

2-1 Cascaded Modulator

– 20 –

Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

DNTFSlide21

2-1 Cascaded Modulator

– 21 –

Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

E

1

(z) completely cancelled assuming perfect matching between the modulator NTF (analog domain) and the DNTF (digital domain)

A 3rd-order noise shaping on E

2

(z) obtained

No potential instability problemSlide22

Integrator Noise–

22 –Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

Delay ignored

INT1 dominates

the overall noise

Performance!Slide23

References–

23 –Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

B. E.

Boser

and B. A. Wooley, JSSC, pp. 1298-1308, issue 6, 1988.B. H. Leung et al., JSSC, pp. 1351-1357, issue 6, 1988.T. C. Leslie and B. Singh, ISCAS, 1990, pp. 372-375.B. P. Brandt and B. A. Wooley

, JSSC, pp. 1746-1756, issue 12, 1991.

F. Chen and B. H. Leung, JSSC, pp. 453-460, issue 4, 1995.

R. T. Baird and T. S.

Fiez

, TCAS2, pp. 753-762, issue 12, 1995.

T. L. Brooks et al., JSSC, pp. 1896-1906, issue 12, 1997.

A. K.

Ong

and B. A.

Wooley

, JSSC, pp. 1920-1934, issue 12, 1997.

S. A.

Jantzi

,

K. W. Martin, and A.S.

Sedra

, JSSC, pp. 1935-1950, issue 12, 1997.

A. Yasuda,

H.

Tanimoto

, and T. Iida,

JSSC, pp. 1879-1886, issue 12, 1998.A. R. Feldman, B. E. Boser

, and P. R. Gray, JSSC, pp. 1462-1469, issue 10, 1998.H. Tao and J. M. Khoury, JSSC, pp. 1741-1752, issue 12, 1999.

E. J. van der Zwan et al., JSSC, pp. 1810-1819, issue 12, 2000.I. Fujimori et al., JSSC, pp. 1820-1828, issue 12, 2000.

Y. Geerts, M.S.J. Steyaert, W. Sansen,

JSSC, pp. 1829-1840, issue 12, 2000.Slide24

References–

24 –Data Converters Oversampling ADC Professor Y. Chiu

EECT 7327

Fall 2014

T. Burger and Q. Huang, JSSC, pp. 1868-1878, issue 12, 2001.

K.

Vleugels, S. Rabii, and B. A. Wooley, JSSC, pp. 1887-1899, issue 12, 2001.S. K. Gupta and

V.

Fong,

JSSC, pp. 1653-1661, issue 12, 2002.

R.

Schreier

et al., JSSC, pp. 1636-1644, issue 12, 2002.

J. Silva et al., CICC, 2002, pp. 183-190.

Y.-I. Park et al., CICC, 2003, pp. 115-118.

L. J.

Breems

et al., JSSC, pp. 2152-2160, issue 12, 2004.

R. Jiang and T. S.

Fiez

, JSSC, pp. 63-74, issue 12, 2004.

P.

Balmelli

and Q. Huang, JSSC, pp. 2161-2169, issue 12, 2004.

K. Y. Nam et al., CICC, 2004, pp. 515-518.

X. Wang et al., CICC, 2004, pp. 523-526.

A.

Bosi et al., ISSCC, 2005, pp. 174-175.N.

Yaghini and D. Johns, ISSCC, 2005, pp. 502-503.G. Mitteregger et al., JSSC, pp. 2641-2649, issue 12, 2006.R.

Schreier et al., JSSC, pp. 2632-2640, issue 12, 2006.