Design and Operation

of Delta-Sigma ADCs

ECE614 Advanced Analog IC Design

Joshua Nekl

May 5, 2008

What is a Delta-Sigma ADC

� AKA Sigma-Delta ADC, over-sampling ADC,

noise-shaping ADC, 1-bit ADC

� Integrates (Sigma) the difference (Delta)

between the quantized output and the input

∑∆+

-

QIN OUT

∆∑ ADC

Analog Digital

[1]

Benefits� No precision matched analog components

� High achievable accuracy

� Low-pass filtering

Drawbacks� Slower conversion time

� DC input current / resistive input impedance

Simple Modulator

Modulator Output

[2]

[2]

0

1

0 1

Input (code)

Output (voltage)

Feedback DAC Transfer Curve

[1]

[2]

1-bit ADC

Quantizer

Offset Cancellation

[1]

Comparator Decision Errors

Noise Shaping

s

A( ) =+− eQ

s

AOUTINOUTIN−

++

-

IN OUT++

( )s

AOUTIN−

Qe

434 214 34 21NTF

e

STF

As

AsQ

AsINOUT

11

1

++

+=

+

0

0.2

0.4

0.6

0.8

1

0.001 0.1 10 1000 100000

STF

NTF

434 21434 21NTF

e

STF

As

AsQ

AsINOUT

11

1

++

+=

Low Pass Filter

noise

2nd order Modulator

s

A1+

+

-

IN OUT++

Qe

s

A2+

+

-

444 3444 21444 3444 21NTF

e

STF

AsAAs

AAsQ

AsAAsINOUT

11

1

121

2

21

2

121

2 +++

++=

Comparison of 1st & 2nd order modulator

Low Pass Filter

0

0.2

0.4

0.6

0.8

1

0.001 0.01 0.1 1 10 100 1000 10000 100000

STF 1st

NTF 1st

STF 2nd

NTF 2nd

Speed vs ResolutionAnalog Devices - AD7732

16

17

18

19

20

0 5000 10000 15000

Conversion Rate (Hz)

Resolution (Bits)

Low Pass Digital Filter

Simple Counter

+ regIN OUTreg

counter

accumulator

z-Transform

)]1([...]2[]1[][][ −−++−+−+= Knxnxnxnxny

)...1()()( )1(21 −−−− ++++⋅= KzzzzXzY

ss f

fj

fTjeez

ππ

22

==

11

1

)(

)()(

−

−

−

−==

z

z

zX

zYzH

K

1

1

1

1−

−

−

−⋅

z

z

[2]

Types of Digital Filters

- Higher Order Sinc Filters

- FIR filters

- BiQuad Filters

References

[1] Baker, R. Jacob, CMOS: Circuit Design, Layout, and Simulation

[2] Baker, R. Jacob, CMOS: Mixed-Signal Circuit Design

[3] Analog Devices AD7732 Datasheet

Questions?

∑∆+

-

1-bit

ADCIN OUTDigital

Filter

1-bit

DAC

Design and operation

of folding ADCs

ECE 614 Student Presentation

By Kaijun Li

May 5, 2008

� Miscellaneous ADC

� Interpolating ADC

� Folding ADC

� Folding & Interpolation

� Simulation

� Conclusion

ECE 614 Design and operation of folding ADCs Kaijun Li

Outline

ECE 614 Design and operation of folding ADCs Kaijun Li

Fig. from ref.[2]

Miscellaneous ADC

2N 2BW

PFOM =

Various FOMs

ECE 614 Design and operation of folding ADCs Kaijun Li

Miscellaneous ADC

� Trade-offs between high conversion rate and

high resolution

� Latency concerns

� Complexity of the CMOS circuitry

ECE 614 Design and operation of folding ADCs Kaijun Li

Flash ADC

Undoubtedly the simplest ADC

Fastest conversion

Largest amount of Preamps and comparators

Figure from [1]

Subranging (or Two-step) ADC

ECE 614 Design and operation of folding ADCs Kaijun Li

Less Comparators

2N -> 2 2N/2

Figure from [1]

ECE 614 Design and operation of folding ADCs Kaijun Li

Interpolating ADC

Figure from [4]

ECE 614 Design and operation of folding ADCs Kaijun Li

Interpolating ADC

0 0.5 1 1.5 2 2.5 3-0.5

0

0.5

Vin

/LSB

Vout

Output of Preamps

Out1

Out2

Out1i

Out2i

0 0.5 1 1.5 2 2.5 3-1

-0.5

0

0.5

1Out2-Out1i

Figure from [2]

Less Comparators

Same amount of Latches

Higher order resistive interpolation

ECE 614 Design and operation of

folding ADCs Kaijun Li

Figure from [5]

ECE 614 Design and operation of folding ADCs Kaijun Li

Transfer curves

Coarse ADC

Fine ADC

Vin

Folder

Latches

MSB

LSB

Digital

Output

DN-1 ,…D1 ,D0

Vin

Vout

11

10

01

00

ECE 614 Design and operation of folding ADCs Kaijun Li

Realization of Folds via Source-Couple Pairs

Figure from [2]

Zero-xing

ECE 614 Design and operation of folding ADCs Kaijun Li

Folding & Interpolation

More practical, they are

combined for application

Figure from [2]

ECE 614 Design and operation of folding ADCs Kaijun Li

LTSpice Simulation

ECE 614 Design and operation of folding ADCs Kaijun Li

� Flash ADC, subranging (or two-step) ADC

� Interpolating and folding techniques

� Interpolating and folding ADC

� Simulation for illustration

Conclusion

ECE 614 Design and operation of folding ADCs Kaijun Li

References

� [1] R. J. Baker, CMOS Circuit Design, Layout, and Simulation, Revised

Second Edition, Wiley-IEEE, 2008

� [2] http://webcast.berkeley.edu/EE 247

� [3] http://www.analog.com

� [4] Willy M.C. Sansen, Analog Design Essentials, Springer, 2006

� [5] H. Kimura et al, “A 10-b 300-MHz Interpolated-Parallel A/D Converter,”

JSSC April 1993, pp.438-446

ECE 614 Design and operation of folding ADCs Kaijun Li

Questions