slides_msc08

Systematic Design for a Successive

Approximation ADCMootaz M. ALLAM

M.Sc– Cairo University - Egypt

Supervisors

Prof. Amr Badawi

Dr. Mohamed Dessouky

• Background

• Principles of Operation

• System and Circuit Design

• Case Study

▫ Simulations

▫ Layout Generation

▫ Performance Evaluation

• Conclusion

• Perspectives

Outline

2

• Emerging new Applications▫ MEMS Sensor Interface:

Resolution: 7-8 bits, BW=50kHz [Scott 2003]

▫ Multi-standards RF receiverResolution: 8 bits, BW = 20 MHz [Montaudon 2008]

▫ Ultra Wide Band (wireless UWB): Resolution: 5-6 bits, BW=300MHz [Chen 2006]

3

The Successive Approximation ADC

« The Return»

Moderate Resolution

Low Power

Minimum Active blocs

Reconfigurable

Figure of Merit

ResolutionFOM

2 *2*

P

BW

4

Objectives

5

▫ Develop a systematic design method for successive approximation ADC from system to layout level .

▫ Develop a general simulation environment with different levels of abstraction and programmed performance analysis.

▫ Emphasis on analog design automation and reuse techniques:

Automatic sizing

Layout generation

▫ Optimizing Layout for best matching

Principle of Operation

6

MSB

LSB1 1 0 0Comp

1 2 3 42 4 8 16

REF REF REF REFV V V VVin b b b b

Outline

• Background

• Principles of Operation

• System and Circuit Design

• Case Study

▫ Simulations

▫ Layout Generation

▫ Performance Evaluation

• Conclusion

• Perspectives

7

Single Ended SAR-ADC.

8

2

REFV REFV

REFV

REFV

inv

8C 4C 2C C C Clock

sample

sample

Sampling Mode

9

REFV

REFV

inv

8C 4C 2C C C Clock

sample

samplesample

invert

Clock

2

REFV Re fV

REFV

VDAC

VDACVIn

VIn

Inversion Mode

10

sample

invert

Clock

gnd

REFV

REF INV v

8C 4C 2C C C Clock

invertREFV

VDAC

REFV vin

VDAC =

INv

Charge redistribution mode (MSB)

11

sample

invert

ClockREFV

2

REFREF

VV vin

8C 4C 2C C C Clock

REFV2

REFINITIAL

VV8C

8C

REFV

REFV

2

REFV

1

VDAC=

VDAC

Charge redistribution mode (MSB-1)

12

sample

invert

ClockREFV

2 4

REF REFREF

V VV vin

8C 4C 2C C C Clock

REFV4

REFINITIAL

VV

4C

12C

REFV

REFV

4

REFV

1 0

VDAC =

VDAC

Mode Redistribution de la charge (MSB-2)

13

sample

invert

ClockREFV

2 8

REF REFREF

V VV vin

8C 4C 2C C C Clock

REFV

REFV

8

REFV

REFV

1 0 1

VDAC =

VDAC

1 2 3 42 4 8 16

REF REF REF REFV V V VVin b b b b

Mode Redistribution de la charge (LSB)

14

sample

invert

ClockREFV

2 8 16

REF REF REFREF IN

V V VV v

8C 4C 2C C C Clock

REFV

REFV

16

REFV

REFV

1 0 1 0

VDAC =

VDAC

1 2 3 42 4 8 16

REF REF REF REFV V V VVin b b b b

Problem

15

ddV

Sometimes DAC ddV V

8C 4C 2C C C Clock

ddV

ddV

ddV

Leakage when using normal PMOS switch.

Possible Solution: Switched charge-pump [scott03] or Bootstrap [dessouky01]

Selecting To increase the dynamic rangeREF ddV V

DACV ddV

ddV

ddV

Possible solutions - Leakage

16

Bootstrap Switch Charge pump Switch

0 < VDAC < 1.5 VDD

VDAC

VDAC

VDD

Since sometimes VDAC value=1.5 VDD

While VG1 = 0 when M1 is ON, VGD,M1 exceeds VDD

Reliability problem

Possible solutions - Reliability

17

0 < VDAC < 1.5 VDD

Shielding Switch

Max OFF = VDD - Vtn

Possible solutions - Circuitry

18

Bootstrap [dessouky01] Modified Shielding

Bootstrap

Differential SAR-ADC

19

2

ddV ddV2

inv

4C 2C C CClock

sample

sample

2

inv

4C 2C C C

2

ddV ddV

2

ddV

2

ddV

Triple Reference

Differential SAR-ADC

20

sample

invert

Clock

2

ddV

DAC1

In1

2

ddV

4C 2C C CClock

4C 2C C C

DAC2

ddV

In2

2

ddV

2 2 4 8 16

dd dd dd ddV V V Vvin

2 2 4 8 16

dd dd dd ddV V V Vvin

1 0 1 0

ddV

ddV 2

ddV

4C 2C C CClock

ddV

Operation – Summary

21

Single Ended Double reference Differential Triple reference

2 times the numbers of capacitors6 times the numbers of switches

Special Switch (charge-pump - bootstrap)

No need for special switch

Differential architectures advantages : - Suppressing even harmonics-Common mode rejection-Offset removal

Lower power consumption Better performance at high frequencies

Outline

• Background

• Principles of Operation

• System and Circuit Design

• Case Study

▫ Simulations

▫ Layout Generation

▫ Performance Evaluation

• Conclusion

• Perspectives

22

Design - Architecture

23

Capacitor array

Comp

ControlSAR

Clock

Switches Control

Switches

In

Sample invert

Capacitor array

Switches

Capacitor array design issues - Noise

24

ddV

ddV

inv

8C 4C 2C C C Clock

sample

sample

C increases, thermal noise decreasesUnit

16TOTC C

ddV

SwitchRinv

1- Thermal noise [ ], due to SamplingTOT

kT

C

Capacitor array design issues - Mismatch

25

2 Capacitor Mismatch (Introduced in fabr ication)

- Affects Generated comparison levels of the capacitve DAC

2

ddV8( )C C

8C

ddV

C increases, mismatch effect decreasesUnit

Capacitor array design issues -

26

16TOTC C

ddV

SwitchRinv

Switch TotalR C

sample

Clock

For an accurate sampling2

Clocksampling

Tt

Samplingf

3- Sampling Frequency

C decreases, bandwidth increasesUnit

Switches

27

• NMOS to switch Vgnd and Vcm

• PMOS to switch Vdd

• CMOS to switch Vin

• Bootstrap to force deep off-state of critical switches

1) Switches selection

2) Sizing switches (compromise)

• Increasing W/L reduces Rswitch and so , on the account of

increasing switch parasitcs.

• In the used DAC, this will be of minor importance if operating in

low frequency because the switches are all connected to the

bottom plates

Design - Architecture

28

Capacitor array

Comp

ControlSAR

Clock

Switches Control

Switches

In

Sample invert

29

ep

Qm

h h

hh

em

MP7 MP8

MN5 MN6

MN1 MN2

MP9 MP11

MN3 MN4

Qp

Cuts the current path

in the RESET phase

Reset to Vdd

Comparator Circuit

Input Signal

Latch

30

Comparison phaseReset phase to Vdd

Latch starts Inputs

Comparator – Operation phases

Resolution > 2

LSBV

Response time < 0.5 TH

31

Sizing input pair and latch

Improve with decreasing L

Tradeoff

Comparator – Design tradeoff

Improve with increasing L

initlatchTotalI offsetV

Design - Architecture

32

Capacitor array

Comp

ControlSAR

Clock

Switches Control

Switches

In

Sample invert

33

LFSR: Linear Feedback Shift Register

DAC outputs C

0 0 0 0 0 0 0 0 0 X

1 1 0 0 0 0 0 0 0 a8

2 a8 1 0 0 0 0 0 0 a7

3 a8 a7 1 0 0 0 0 0 a6

4 a8 a7 a6 1 0 0 0 0 a5

5 a8 a7 a6 a5 1 0 0 0 a4

6 a8 a7 a6 a5 a4 1 0 0 a3

7 a8 a7 a6 a5 a4 a3 1 0 a2

8 a8 a7 a6 a5 a4 a3 a2 1 a1

SAR algorithm - Implementation

Systematic design for SA-ADC

34

Resolution – BW – Power - Techno

Non idealities and noise Thermal noise, mismatch, …

Cmax >Cmin

Switches Sizing

Yes

No

Clock frequency

Cmax

C unity = C min

Sizing procedure

System architectureDAC topology, SAR algorithm, Sampling technique

Cmin

Comparator specsSettling, resolution, kickback….

Select number of stages

Check Specs

Layout

No No

Yes

Finalize design

No

Outline

• Background

• Principles of Operation

• System and Circuit Design

• Case Study

▫ Simulations

▫ Layout Generation

▫ Performance Evaluation

• Conclusion

• Perspectives

35

Case Study

36

• Case Study▫ Differential Architecture

▫ Resolution: 8bit

▫ BW: 50 KHz

▫ Fclock: 1MHz

▫ Technology: 0.13u ST, MIM Capacitors

• Verification▫ VHDL AMS used for verification with simulation

▫ Different levels of abstraction (Behavioral , gates, transistor, …)

▫ Mixed blocs simulation (Analog / Digital)

Multiple abstractions

37

Capacitor array Comp

+ Latch

ControlIn

Switches

Macro

model

Macro

model Transistor

Transistor

MIM

VHDLMacro

model

Clock gen VHDLTransistor

Verification Environment

38

Resolution – BW

Sketch output spectrum

Verification Environment presetsDAC topology, SAR algorithm, Sampling technique

Abstraction levelIdeal, mismatched, techno, …

Component sizes

Type of analysis

Calculate SNDR

Sketch SNDR v.s. fin

Sketch SNDR v.s. Ain

Sketch INL and DNL

Sketch Transient response at each node

Transiant – Single Ended - Output

39

Vdd 1.2V

Fin 1.4KHz

Fclk 1MHz

Vinp-p 1.2V

Transistor level simulations

Transiant – Differential - Output

40

Vdd 1.2V

Fin 1.4KHz

Fclk 1MHz

Vinp-p 1.2V

Transistor level simulations

Transiant – Differential - DACs

41

Vdd 1.2V

Fin 1.4KHz

Fclk 1MHz

Vinp-p 1.2V

Differential DACs outputTransistor level simulations

Transiant – Differential - Comparator

42

Vdd 1.2V

Fin 1.4KHz

Fclk 1MHz

Vinp-p 1.2V

Full conversion: Differential DACs output – Comparator outputTransistor level simulations

Transient – SAR control

43

Vdd 1.2V

Fin 1.4KHz

Fclk 1MHz

Vinp-p 1.2V

VHDL DescriptionControl block turning ON and OFF DAC switches [case of 0 input]

Transient – Full Scale Ramp

44

Full Scale Slow ramp excitation

Vinp-p 1.2V

Zoom - in

Static Performance - Transistor Level

45

Static performance Evaluation in (LSB): DNL and INL [16 sample / bin]

Dynamic Performance – Ideal Models

46

Vdd 1.2V

Fin 1.4KHz

Fclk 1MHz

Vinp-p 1.2V

SNDR = 47.47 dB

4096 point FFT

Ideal models simulation

SNDR = 47.44 dBSNDR = 47.47 dB

SNDR = 46.78 dB SNDR = 46.69 dB

Dynamic Performance – Mixed Models

47

Vdd 1.2V

Fin 1.4KHz

Fclk 1MHz

Vinp-p 1.2V

4096 point FFT

Ideal MOS

switches

MOS

comparator

0.01

mismatch

in Cu

Dynamic Performance – Transistor Level

48

Vdd 1.2V

Fin 1.4KHz

Fclk 1MHz

1024 point FFT

Pow

er

Specra

l D

ensity (

dB

)

Frequency (Hz)

SNDR = 46.2 dB

Transistor level simulations

49

Vdd 1.2V

Fin 1.4KHz

Fclk 1MHz

SNDR max

Ideal =47.47 dB

Transistor Level = 46.2

Sig

nal to

nois

e a

nd d

isto

rtio

n r

atio S

ND

R(d

B)

Amplitude of input signal(dB)

4096 point FFT

Transistor level simulations

Dynamic Performance

50

Vdd 1.2V

Fclk 1MHz

Vinp-p 1.2V

BW =55 KHz

Sig

nal to

nois

e a

nd d

isto

rtio

n r

atio S

ND

R(d

B)

4096 point FFT

Transistor level simulations

Dynamic Performance

Frequency of input signal (Hz)

Mismatch analysis

51

Vdd 1.2V

Fin 1.4KHz

Fclk 1MHz

Vinp-p 1.2V

4096 point FFT

0.1

mismatch

in Cu

0.05

mismatch

in Cu

SNDR = 46.36 dB SNDR = 44.13 dB

Layout generation for SA-ADC

52

Comparator transistor sizes

Unit capacitance

Common centroidplacement algorithm

Desired layout shape

Layout template s-Component connectivity-Relative place and route

CAIRO Layout generation

DRC – LVS

Design phaseNumber of

capacitors and sizes

Target technology

Verification

Parasitics Ext.

Fabrication

Layout – Comparator - Floorplan

53

Layout – Comparator - Generated

54

Dummies Removed for Layout verificationArea 22 x 39 µm2

Layout – Differential DACs - Floorplan

55

Common centroid placement for 16 capacitor

Layout – Differential DACs - Generated

56

Layout 1 - 256 Cu – Placed and Routed – Area

1.26 x 0.26 mm2 and Huge routing parasitics

Layout – Differential DACs - Manual

57

Layout 2 - 256 Cu – Placed

and Routed – 2/3 less

routing parasitics

ZOOM

Area 0.1056 mm2

Performance

58

[Hong07] This work* [scott03]

Technology 0.18 µm 0.13 µm 0.25 µm

Supply 0.83 V 1.2 V 1.0 V

Input range Rail to Rail Rail to Rail Rail to Rail

Sampling rate 111 KHz 111 KHz 100 KHz

Unit Cap. 24 fF 30fF 12f

Power (Analog) 1.16 µW 0.72µW 2.2 µW

Area 0.062 mm2 0.122 mm2 0.053 mm2

SNDR@BW 47.40 dB 46.2dB 43.8 dB

Architecture Single Ended Differential Single Ended

FOM 65 fJ/bit 64fJ/bit 2163 fJ/bit

Outline

• Background

• Principles of Operation

• System and Circuit Design

• Case Study

▫ Simulations

▫ Layout Generation

▫ Performance Evaluation

• Conclusion

• Perspectives

59

Summary and Conclusion

60

▫ Systematic design methodology for SA-ADC from

system to layout.

▫ General simulation environment

▫ Different abstraction levels.

▫ Different verification tests.

▫ Emphasis on analog design automation and reuse

▫ Optimizing Layout for best component matching

▫ Verification with case study for WSN specs

Perspectives

61

▫ Targeting high frequency specs (>500 Msample/S) Redundant system error correction code [Kuttner02]

Digital calibration [Promitzer01]

Asynchronous operation [Chen06]

Time interleaving [Chen06]

▫ Full Automation Sizing procedure with layout parasitics awareness

Layout generation for the full ADC

Thank You

62