Mixed signal systems and integrated circuits · 5/10/2009 A. Matsuzawa 3 Aim of this lecture • Understanding basic current mixed signal systems – Wireless transceiver • Understanding

5/10/2009 A. Matsuzawa 1

Mixed signal systems and integrated circuits

Akira Matsuzawa

Tokyo Institute of Technology


Contents

• Mixed signal systems• High speed A/D converters• High speed D/A converters• Sigma delta A/D and D/A converters• Wireless systems and RF CMOS circuits• PLL and related systems


Aim of this lecture

• Understanding basic current mixed signal systems– Wireless transceiver

• Understanding basic mixed signal circuit building blocks: basic operation method and basic design method

– A/D and D/A converter– Sigma-delta modulation– Phase Lock Loop and Delay Lock Loop– Low Noise Amplifier– Frequency Mixer– Voltage Controlled Oscillator and Frequency Synthesizer


1. Mixed signal systems


Current electronics andmixed signal technology


Exciting digital consumer electronics world

SemiconductorTechnology

MediaProcessorBetter Look

Better SoundHigher Quality

Anywhere

ExcitingMultimedia

with System LSISolutions

ExcitingMultimedia

with System LSISolutions

Audio and Video

BroadcastingCommunication

Network

System and SoftwareTechnologies

Storage Media

Anytime

New consumer electronics era has been emerged.Key technologies are digital multimedia and System on a Chip.


LCD Driver

LCD Drivers

LCD driver is a simple example of mixed signal LSI


LCD Driver

Start

Shift Resigter 64

Flip-Flop

384 * 6 bits Latch

384 * 6 bits Latch

6bits*3=186bits*3=18

Carry

6bits*3=18

384 * Voltage Scalling DA Converter

D00-07D20-27D40-47

Flip-Flop

D10-17D30-37D50-57

384 * 6 bits level shifter

sele

ctor

1 64

6

6

6

6

6 6

6

6

Cotroler6bit *R,G,B*2=36bit

384 output

#1 #2 #8

XGA: 1024*RGB (=3072) → 3072/384=8LSIs

subpixel pixel

LCD driver is an array of DA converters


Home Home ServerServer

NetworkNetwork

ITSITS

CS/BSCS/BS

WW--CDMACDMA

HII StationHII Station

DVDDVDDVCDVC

Digital TVDigital TV

Image of current electronicsDigital consumer electronics and networking drive current electronics.

ADSL, FTTH

DAB

Digital TV

Home network

Ethenet

IEEE 1394, USB, Blue tooth, Wireless LAN


Mixed signal technology :Digital networkingｓ

Side-streamDescramber

&Trellis, Viterbi decoder

DACDACDACDAC

250Mbaud (PAM-5)

ADCADCADCADC

3-NEXTCanceller

Echo Canceller

DFE

Slicer

Clock Recovery

FFE

TX1TX2

TX3TX4

Pulse Shaping

Side-streamScramber

&Trellis,Viterbi

Symbol EncoderLine

I/F 6b, 125MHz ADC, DAC

Analog circuit

Mixed signal technology enables high speed digital networking.

Digital circuit

Error correction

Equalization Encryption

Noise cancellation

Digital

Data and clock recovery

Data conversion

Analog


x-DSL

ADSL:0.1MHz-1.1MHZVDSL:2.0MHz-3.5MHz

○ADSL-service, 0.5-8Mbps(Dwn)/1Mbps (Up) for 5～6Km○VDSL-service, 13-52Mbps(Dwn) for 0.3-1.5Km ,

RX-in

Anti-aliazingFilter

ADCAdaptive DFE

Deci-mationFilter

DDFS

Error CorrectionFEC

TX-out

ReconstractionFilter

DAC

Inter-polationFilter

DDFS

Error CorrectionFEC

○ 96-tap Decision Feedback Equalizer(DFE)

○ 4～256-QAM modulation→ 60MHz 10-bit ADC and DAC for VDSL→ 5MS/s 14-bit ADC and DAC for ADSL

○ T=8 Read-Solomon Forward Error Correction (FEC)

ADSL and VDSL use the mixed signal technology


Mixed signal tech. ; Digital read channel

Variable Gain Amp.Variable

Gain Amp.Analog

FilterAnalog

FilterA to D

ConverterA to D

ConverterDigital

FIR FilterDigital

FIR FilterViterbiError

Correction

ViterbiError

Correction

ClockRecoveryClock

RecoveryVoltage

ControlledOscillator

Voltage ControlledOscillator

DataOut

Data In(Erroneous)

Data Out(No error)

Analog circuit

Digital circuit

Digital storage also needs high speed mixed signal technologies.

Pickup signal


Mixed signal SoC for DVD RAM system

0.18um- eDRAM

24M Tr16Mb DRAM

500MHzMixed Signal

Goto, et al., ISSCC 2001

This enables high readability for weak signal from DVD RAM pickup.

World fastest and highly integrated mixed signal CMOS SoC


Mixed signal SoC

PixelOperationProcessor


IOProcessor

IOProcessor

AVDecode

Processor

AVDecode

Processor

Back -EndBack -End

SystemCont-roller

SystemCont-roller

CPU１CPU１CPU2CPU2

VCOVCO

ADCADC

Gm-CFilterGm-CFilter

PRMLRead

Channel

PRMLRead

ChannelServo DSPServo DSP

AnalogFront EndAnalog

Front End

Front-EndFront-EndAnalog FE+Digital R/C

0.13um, Cu 6Layer, 24MTr

Okamoto, et al., ISSCC 2003

Mixed signal SoC can realize full system integration for DVD application. Embedded analog is the key.


Recent developed mixed signal CMOS LSIs

5G RF LAN 12b 50MHz ADC 2ch12b 50MHz DAC 2ch

AFE for ADLS 12b 20MHz ADC+DAC

Digital network1394b (1GHz)

AFE for Digital Camera12b 20MHz ADC+AGC

2GHz RF CMOS

AFE (Analog Front End)


Application area in mixed signal CMOS tech.

NetworkCommunication

NetworkCommunication

RecordingRecording

OutputOutput

InputInput

・Cellular phone: PDC, W-CDMA・RR-Net: Bluetooth, IEEE802.11・Broad cast: STB, DTV, DAB

・Optical:FTTH, OC-xx・Metal: ADSL, VDSL, Power line modem

・Serial: IEEE1394, USB, Ethernet・Parallel: DVI, LVDS

・DVD, VDC, HDD

Wireless

Wired

・LCD, PDP, EL, Audio drive

・Camera, Others

Power supplyPower supply ・ Switching supply, Every LSIs (On-chip)

Almost all the products need mixed signal CMOS LSI tech.


Digital technology in real world

• High robustness• Programmability• Time shift (memory)• Error correction• High Scalability

Pure digital

Media(Cable, Disc, Air, etc) Real world

Damaged digital

Recovered digital

Advantages of Digital Tech.

Mixed signal technology(Analog+Digital)

Mixed signal technology(Analog+Digital) Reconstruction

But, digital can address this issue by own advantages,but needs the help of analog tech.

NoiseDistortionInterferenceLimited bandwidth

Not only digital, but also analog;ADC, DAC, Filter, and PLL are needed

Digital signal suffers heavy damage in real world.


Role of current analog technology

InterfaceDigital signalProcessingand control

Powersupply

ClockGeneration

Outer world

Energy conversion

Wireless com.

Wired com.

Recording

Image

Audio

Motor

Sensor

Analog： Physical aspectsDigital： Meta-physics

(Brain）

(Sense and actuate organ; Mouse, Eye, Ear, Nose, etc.)

(Digestive organ, Circulatory organ)

The role of current analog technology is an interface between digital technology and outer physical world. Analog supports digital.


Basic technology for digital network and storage

AnalogProcessing

AnalogProcessing

DataConverter

DataConverter

Communicationprocessing

Communicationprocessing

Data compression

Data compression

NetworkStorage media

・Mod/ Demod・Channel select・Error correction・Protocol・Encryption

・MPEG2, 4・DSP・Codec

・A/D Converter・D/A Converter

・RF・Optical I/F・Cable drive・Signal Generation

Analog technology Digital technology

Analog and data converter technologies are needed for digital network and digital storage


'85 '90 '951

10

100

50

20

5

2

Perf

orm

ance

Inde

x N

umbe

r

Perfec TV

DVCApplied System

6b,800MHz

8b,120MHz

10b,20MHz

10b, 30MHz

8b,20MHz

10b, 300MHz

10b, 20MHz,30mW

HDTV

Camera

MUSE Receiver

Video Camera

Wide-TV

HDTV

DigitalCamera

6b, 80MHz

8b, 100MHz

Video Switcher

Digital OSC

Digital OSC

Development of ADCs for digital consumer products

DVDBip / BiCMOS

CMOS

6b, 1GHz

Development of ADCs has contributed to the progress of digital consumer electronics.


Progress in A/D converter; video-rate 10b ADC

1980 1982 1993 Now

Board Level (Disc.+Bip)20W

$ 8,000

Conventional product World 1st Monolithic

Bipolar (3um)2W

$ 800

World lowest power

CMOS (1.2um)30mW$ 2.00

CMOS (0.15um)10mW$0.04

SoC Core

ADC is a key for mixed signal technology.We have reduced the cost and power of ADC drastically;1/ 2,000 in Power and 1/200,000 in cost!

CMOS technology attained it.

Analog Devices Inc.

dulling past 20 years

Our development

Our development

Our development


1

10

100

1000

10000

1980 1985 1990 1995 2000 2005 2010Year

Pow

er (m

W)

2

5

20

50

200

500

2000

5000 FlashTwo-stepSubrangingFolding/InterpolatingPipeline

OthersLook-ahead Pipeline

1

10

100

1000

10000

1980 1985 1990 1995 2000 2005 2010Year

Pow

er (m

W)

2

5

20

50

200

500

2000

5000 FlashTwo-stepSubrangingFolding/InterpolatingPipeline


FlashTwo-stepSubrangingFolding/InterpolatingPipeline


0.1

1.0

10.0

100.0

1980 1985 1990 1995 2000 2005 2010Year

Are

a si

ze (m

m2)

0.2

0.5

2.0

5.0

20.0

50.0FlashTwo-stepSubrangingFolding/InterpolatingPipeline


0.1

1.0

10.0

100.0

1980 1985 1990 1995 2000 2005 2010Year

Are

a si

ze (m

m2)

0.2

0.5

2.0

5.0

20.0

50.0FlashTwo-stepSubrangingFolding/InterpolatingPipeline




Power reduction Area reduction

Power and area reduction of video-rate 10b ADCs

Power and area of ADC have been reducing continuously.Currently, ADC can be embedded on a chip


0.1

1.0

10.0

100.0

0.1 1 10

Process node (m)

Pow

er/M

Hz

(mW

/MH

z)

0.2 0.3 0.5 0.7 2 3 5 7

0.2

0.5

2.0

5.0

20.0

50.0



0.1

1.0

10.0

100.0

0.1 1 10

Process node (m)

Pow

er/M

Hz

(mW

/MH

z)

0.2 0.3 0.5 0.7 2 3 5 7

0.2

0.5

2.0

5.0

20.0

50.0





0.1

1.0

10.0

100.0

0.1 1 10Process node (m)

Are

a si

ze (m

m2)

0.2 0.5 2 5

0.2

0.5

2.0

5.0

20.0

50.0



0.3 0.70.1

1.0

10.0

100.0

0.1 1 10Process node (m)

Are

a si

ze (m

m2)

0.2 0.5 2 5

0.2

0.5

2.0

5.0

20.0

50.0





0.3 0.7

M. Hotta et al. IEICE 2006. June

Power and area reduction of video-rate 10b ADCs


Early stage mixed signal CMOS LSI for CE

6b Video ADC

8b low speed ADC;DAC

Digital Video filter

8b CPU

1993 Model: Portable VCR with digital image stabilizing

Success of CMOS ADC and DAC enabled low cost mixed signal CMOS LSI.This also enabled low cost and low power digital portable AV products.

System block diagram


Mixed signal system: Digital Camera

Current camera system uses digital technology.


Ultra-high speed ADCs

8b, 120MHz, (1984）

World fastest 8b ADC

8b, 600MHz ADC (1991）


6b, 1GHz ADC (1991）

World fastest in production(Dual Parallel method）

Ultra-high speed ADCs have been developed.


Digital Oscilloscope

松下通信工業：10b 100MHz OSC (1986年）

横河電機： 8b 1GHz (1994年）

Ultra-high speed ADCs have realized Digital Oscilloscopes.


Progress in high-speed ADC

0.1

10

Pd/2

N[m

W]

Reported Pd of CMOS ADCs

Conversion rate [x100Msps]

1

1mW/Gsps

10mW/Gsps

This Work

101

1 order down

6b, 1GHz ADC2W,1.5um Bipolar

6b, 800MHz ADC400mW, 2mm2

0.25umCMOS

7b, 400MHz ADC50mW, 0.3mm2

0.18umCMOS

ISSCC 2002

ISSCC 2000

ISSCC 1991

World fastest CMOS ADC

World lowest Pd HS ADC


High speed ADC has reduced its power and area down to be embedded.


System: DVD player

ConsoleConsolePanelPanel

HeadHeadAmpAmp

DemodulationDemodulationECCECC

ACAC--3 Output3 Output

MPEG 2MPEG 2VideoVideo

ACAC--3 Audio3 Audio

System ControllerSystem ControllerMCUMCU

CDCDDEMDEM

16M16MSDRAMSDRAM

DriverDriver

Optical DiscOptical Disc Optical Optical HeadHead

Pre AmpPre AmpStereo OutputStereo Output

Video OutputVideo Output

CopyCopyProtectionProtection

PhotoPhoto--receptivereceptiveCompoundCompound

ServoServoDSPDSP

AnalogAnalogFront EndFront End

ODCODC

AV DecoderAV DecoderRed Laser UnitRed Laser Unit

Servo DSPServo DSP System Controller MCUSystem Controller MCU

4M4MDRAMDRAM

Red LaserRed Laser

：：FirstFirst--Gen.Gen.

：：SecondSecond--Gen.Gen.

：：ThirdThird--Gen.Gen.

：：FourthFourth--Gen.Gen.

ReadReadChannelChannel

OSAPI

High-speedAnalog-Digital

32bit MCUDRAM Embedded Media Core

Processor

MPEGAlgorithm

Analog

Memory

Current electrical system is complicated and needs analog and memory.


Full DVD system integration in 0.13um tech.



IOProcessor

IOProcessor

AVDecode

Processor

AVDecode

Processor

Back -EndBack -End

SystemCont-roller

SystemCont-roller

CPU１CPU１CPU2CPU2

VCOVCO

ADCADC

Gm-CFilterGm-CFilter

PRMLRead

Channel

PRMLRead

ChannelServo DSPServo DSP

AnalogFront EndAnalog

Front End

Front-EndFront-EndAnalog FE+Digital R/C

0.13um, Cu 6Layer, 24MTrOkamoto, et al., ISSCC 2003

Advanced mixed signal SoC has been successfully developed.


Cost reduction in DVD Recorder

’2000 Model

’2003 Model

One-chip integration for hole DVD system has been realized.This makes circuit board simpler and contribute to the cost down,as well as performance up.


Scaled CMOS technology

Seven latticesGate

Si

SiO2

100nm

Transistor Cu Interconnection

Current Scaled CMOS technology is very artistic.

Matsushita’s 0.13um CMOS technology


CMOS as analog device

--++Embed in CMOS

--++Digital calibration

--++Analog calibration

--++Offset cancel

+-Low Sub. effect

CMOS is 10x to 100x of Bip.++--1/f noise

CMOS is 10x of Bip.++--Voltage mismatch

Almost same++fT

This results in Cp issue-+Low Capacitance

CMOS is ¼ of Bip.+-High gm

--++Low Input current

--++Switch actionCommentBipolarCMOS

Only CMOS can realizeswitched capacitor circuits

CMOS has a variety of techniquesto address the self issues

CMOS has many issues as analog device,but also has a variety of circuit techniques


GHz operation by CMOS

inT

Cgmf2

Cutoff frequency of MOS becomes higher than that of Bipolar.Over several GHz operations have attained in CMOS technology

1995 2000 2005

1G

10G

100G

100M

Freq

uenc

y (H

z)

200M

500M

2G

5G

20G

50GfT : Bipolar (w/o SiGe)

fT

Year

eff

satTpeak

Lvf2

D R/C for HDDIEEE 1394

/60 (CMOS )Digital circuits

fT : CMOS

0.35um

0.25um0.18um

0.13um

fT

CellularPhone

/10 (CMOS )

CDMA

RF circuits

5GHz W-LAN


World first 1394b transceiverFor 1Gbps networking

0.25um 3AL_CMOS

5Gbps Eye pattern

0.18um 4AL_CMOS

Test chip for 5Gbps wire line

Digital consumer needs over GHz wire line networking.CMOS has attained 5Gbps data transfer.

CMOS technology for over GHz networking


Basic issue of analog in LSL technology

RuleDesign　1

tox

L

W

XjLeff

0.7x

Perf

orm

ance

(Log

)

Scaling

(Log)

Integration2

1L

Speed 5.1

1L

Dynamic range =

5.1LNoise + Mismatch+Distortion

Signal swing

Scaling Rule

Scaling can realize higher integration and higher speed yet low power for digital circuits.In contrast, analog performance is used to be degraded with scaling.

Architectural and circuit technology development has been needed.


Wireless systems

PA

NLA

N

UWBBluetoothZigBee

802.11n(100M)

802.11a/g(54M)

IEEE802.11b(11M)

4G

2010

IEEE802.15

A-PHSPHS

HSDPA (14M)

W-CDMA(384k)

PDC

EV-DV(5.2M)

cdma2000-1XEV-DO(2.4M)

cdma20001x(144K)

IEEE802.20(4M)

EDGEGPRSGSM

2005年～

PA

NLA

NC

ellu

lar

UWBBluetoothZigBee

802.11n(100M)

802.11a/g(54M)

IEEE802.11b(11M)

4G

2010

IEEE802.15

A-PHSPHS

HSDPA (14M)

W-CDMA(384k)

PDC

EV-DV(5.2M)

cdma2000-1XEV-DO(2.4M)

cdma20001x(144K)

IEEE802.20(4M)

EDGEGPRSGSM

Data rate

The number of wireless standards are increasing


Technology edge RF CMOS LSI

M. Zargari (Atheros), et al., ISSCC 2004, pp.96 K. Muhammad (TI), et al., ISSCC2004, pp.268

Discrete-time Bluetooth0.13um, 1.5V, 2.4GHz

Wireless LAN, 802.11 a/b/g0.25um, 2.5V, 23mm2, 5GHz

Many RF CMOS LSIs have been developed for many standards


Current status of RF CMOS chip

• Current products– Bluetooth: 2.4GHz, CSR etc., major– Wireless LAN: 5GHz, Atheros etc., major– CDMA : 0.9GHz-1.9GHz, Qualcomm, becomes major– Zigbee: 2.4GHz, not yet, however must use CMOS– TAG: 2.4GHz, Hitachi etc., major

Major Cellular phone standard, GSM uses SiGe-BiCMOS technology

RF CMOS was a university research theme, however currently becomes major technology in wireless world.


Why CMOS?

• Low cost– Must be biggest motivation– CMOS is 30-40% lower than Bi-CMOS

• High level system integration– CMOS is one or two generation advanced– CMOS can realize full system integration

• Stable supplyment and multi-foundries– Fabs for SiGe-BiCMOS are very limited. Slow price decrease and limited product capability

• Easy to use– Universities and start-up companies can use CMOS with low

usage fee, but SiGe is difficult to use such programs.


Multi-standard issue

IMT-2000RF

GSMRF

BluetoothRF

GPSRF

GPSBB

BluetothBB

GSMBB

IMT-2000BB

MCU

Power

ReconfigurableRF DSP

Unification

Yrjo Neuvo, ISSCC 2004, pp.32

Unified wireless system

Multi-standards and multi chipsFuture cellular phone needs 11 wireless standard!!

Reconfigurable RF circuit is strongly needed for solving multi-standard issue.

Current

Future


Scalable circuit design for wireless systems

Scalable and reconfigurable design is needed for addressing the multi-standard wireless systems

Changeable: ADC/DAC resolution and bandwidth


Basics of analog to digital and digital to analog conversion


Basic mixed signal system

DSPADC DACPre

Filter(low pass)

PostFilter

(low pass)AGC

Clock

Time continuous Time discrete Time continuous

Mixed signal systems has DSP, ADC, DAC, and pre/post filter basically.The signals are converted between time continuous and time discrete.


Sampling theory

n

nTttv )( dtetvT

VeVtvT

TTntj

n

n

Tntj

n

2/

2/

22

)(1,)(

-fm +fm

F(x(t))

Time

Volta

gex(t)

Time domain Frequency domain

Time domain Frequency domainT: period 0 fc 2fc 3fc 4fc

Signal

SamplingPulse

1,1 2 njn e

TV

The signal has bandwidth of fm. Periodical sampling pulse has a period of T.

fc=1/T

Fourier expansion

n

Tntj

eT

tv21)(


Sampling

n

nTttv )(

1

)()(n

cc fnfXfnfXfXtvtxFT: period

Time

x

x(t)

Time

x0 fc 2fc 3fc 4fc

X(t)v(t)

nTtnTxtvtxn

)()(

Sampling

Signal

SamplingPulse

Sampling process can be treated as the product of the signal and the sampling pulse

x(t)

v(t)

Frequency

Sampled signals have multi-sidebands at Nfc


Frequency spectrum in sampled data.

nTtnTxtvtxn

)()(2

cos2cos2111)(1

2 jxjx

nn

Tntj eex

Tnt

Te

Ttv

.......23cos222cos22cos211)( tftftfT

tv ccc

.......23cos)(222cos)(22cos)(2)(1)()( tftxtftxtftxtxT

tvtx ccc

Thus x(t)v(t) can be regarded as a AM modulated signal that the career signal of which frequency is nfc and the modulated signal is x(t)

If simply assuming x(t) is single tone: xocos (2πfat)

tftftfTxtvtx c

n

aao 2cos2cos22cos)()(

1

12cos2cos2cos

n

acacao tfnftfnftf

Tx

)cos()cos(21coscos BABABA

Sampled signal has a sideband of +/- fa at around nfc


Signal reconstruction from sampled data

fmfc-fm

fc+fm 2fc+fm2fc-fm

fc 2fc0

fmfc-fm

fc+fm2fc+fm2fc-fm

fc 2fc0

Signal overlap

Signal can not be separated

Signal non-overlap

Signal can be separated to reconstruct

2c

mff

2c

mff

Low pass filter

fc/2

If signal bandwidth is less than fc/2, signal can be reconstructed perfectly.

Nyquist condition

1

)()()(n

cc fnfXfnfXfXtvtxF F(x(t)v(t)): Fourier transform of x(t)v(t)X(f): Fourier transform of the analog signal


Reconstruction from sampled signals

nTtnTxtvtxn

)()(

20

21

)(c

c

G

Sampled signal:

Time

x

Time

x

x(t)0dB

ωc/2

Angular frequency

Ideal Low pass filter

Pass Stop

Sampled signal Reconstructed signal

Ideal Low pass filter:

nTtf

nTtfnTxtyc

c

n

sin)()(

tftftv

c

c

sin)(

Sampled signal can be reconstructed to be continuous signal through low pass filter.

For the unit impulse signal

)()()( nTtvnTxtyn


Reconstruction by sampling function

Original signal

Sampling

再生過程 Reconstruction

Sampling function

nTtf

nTtfnTxtyc

c

n

sin)()(

tf

tftSc

c

sin)(

cfT 1

Signal can be reconstructed by the convolution between sampling signal and sampling function.

)()()( nTtSnTxtyn


Aliasing effect

Frequencies are folded

csigc

sigalias

csigccsigalias

fnffnffnf

fnfnfnfff

12

12:1

212:

Noise

Accumulated Noise

Signals of which frequencies are higher than fc/2 are folded to the lower frequencies L.T. fc/2.

Nose which spreads wide frequency is also folded to lower frequency and accumulated.

Low pass filter is needed

Caution!!

Sampled signal is conventionally Noisy


Special technique: Under sampling

2GHz

Bandwidth is 8MHz

Carrierfc=20MHz

2GHz carrier

20MHz sampling

8MHz signal

Under sampling technique

By using under sampling technique, we can obtain modulated signal from very high carrier frequency.However, very low SNR due to noise accumulation.


Reconstruction process

Interpolated signals

Sampled signals

Reconstructed signals and interpolation

Conversion period

Conversion frequency

Spectrum of reconstructed signals

Spectrum of over sampled signals

Folding noise

Folding noise

Required LPF spec.

Original signal

Reconstructed signals has also folding frequency components.Thus DAC need post low pass filter. The interpolation technique can relax the required LPF spec.


Aperture effect in DAC

Time

x

Time

x

Ideal impulse train Actual Step pulse train in DAC output

DSP DAC

Frequency characteristics of DAC

Sign

al in

tens

ity

c

c

ff

ff

fA

sin

)(

Aperture effect

High frequency signal of DAC is decreased

Use aperture correction filter that has inverse frequency characteristics.

Reduce the pulse width by using small duty pulse

Increase the conversion frequency using over sampling technique

Due to the aperture effect, the higher frequency component of the output signal from DACIs decreased. Sometime some technique is needed.


Frequency spectrums in ADC and DAC

Input signal to ADC

Signal in ADC and DSP

Signal from DAC without the aperture effect

Signal from DAC with the aperture effect

Folding

Re-folding

Aperture effect


Quantization

+

Nominal full-scale

Effective full-scale

Analog input

Dig

ital o

utpu

t

Ideal line

Minimum step (1LSB)

Quantizationerror

Input signal Quantized signal

Quantization noise

Analog to Digital Converter

Quantized signal = Input signal + Quantization noise

0 to 2N-1

0 to 2N

LSB (Least Significant Bit)

Quantization step

Ideal quantization error

ADC has a finite resolution number and the signal is quantized.This causes error called “quantization error”.


Quantization noise and SNR

qx

qxqxp

5.0,0

5.0,1)(

25.0

5.0

2

231)(

qdxxpxNq

qq

2

22

21

qSN

)(76.102.6

)5.1log(102log20log10/

dBNNSSNR N

qrmsrms

Step: q

Full scale:

Ideal quantization error

Signal intensity

qS N2

Probability density of quantization error

Noise power

Signal power

Signal to Noise Ratio

Quantization causes noise and this noise power reduces with increase of resolution number.Principal signal to noise ratio (dB) of N bit ADC is about 6N+2.The higher resolution of ADC realizes the higher SNR for signal processing.


SNR increase by increasing fsc

Frequency

Frequency

fc=10MHz

fc=20MHz

fc/2=10MHz

fc/2=5MHz

Signal=2MHz

Signal=2MHz

fb=5MHz

Half noise powerIs removed

Quantizationnoise

Quantizationnoise

Conversionclock

Conversionclock

NoiseAfter LPF

2x conversion rate

Total Noise power is same,but power density is lower

b

crmsrms

ffNSNR

2log1076.102.6/

fc: Conversion frequencyfb: Bandwidth of LPF

We can increase SNR by increasing of conversion frequency with low pass filter.

3dB higher SNR by 2x higher fc

Documents

Mixed signal systems and integrated circuits · 5/10/2009 A. Matsuzawa 3 Aim of this lecture • Understanding basic current mixed signal systems – Wireless transceiver • Understanding