System Aspects of ADC Design

SHANTHI PAVAN

Assistant Professor

Department of Electrical Engineering

Indian Institute of Technology, Madras

05/18/01 V4.3

Prakash Easwaran, C Srinivasan

Cosmic Circuits

A-to-D Converters Terminology & Architectures

05/18/01 V4.3

PRESENTATION OVERVIEW

• ADC System Overview

• ADC Metrics

• Flash & Folding Converters

• Two Step Flash Converters

• Pipeline & Delta-Sigma ADCs

• Power efficiency of ADCs

• Conclusions

A GENERIC MIXED-SIGNAL SYSTEM

THE A-D CONVERSION PROCESS

• Anti-alias filter limits input bandwidth to fs/2

ADC OPERATIONS : SAMPLING

• Input typically stored

as charge on a

capacitor

• Tracking bandwidth

• Aperture

• Hold pedestal

QUANTIZER BASICS

• Quant. error assumptions– uniformly distributed– uncorrelated with input !– white !

][2

,2

eq

12

22 eq

0eq

OFFSET ERROR

• Benign when relative accuracy is desired

- Cancelled digitally

Ideal

Actual

GAIN ERROR

• Benign when relative accuracy is desired

-Correct using AGC- in the analog/digital domains

Ideal

Actual

DIFFERENTIAL NONLINEARITY (DNL)

• Nonmonotonicity & missing codes • Monotonic if |DNL| <

INTEGRAL NONLINEARITY (INL)

• Measures deviation from a line• |INL| < 0.5 sufficient condition for a monotonic characteristic

DNL & INL REMARKS

• DNL & INL should be measured with the best fit line

for good repeatability

• DNL - picture of local variations in quantizer

thresholds

• INL - picture of long range variations in quantizer

thresholds

DYNAMIC PERFORMANCE METRICS

• Signal to Noise Ratio (SNR)– Signal to quantization noise

• (6N + 1.76) dB for a sine wave • 1/2 the step size means ¼ the noise power

• Signal to Noise + Distortion Ratio (SNDR)– Signal to everything else

• Spurious Free Dynamic Range (SFDR)– Signal to the largest spectral spur

• Effective Number of Bits (ENOB)–

02.6

76.1SNDR

SPURIOUS-FREE DYNAMIC RANGE

Input Tone

Quantization Noise

HARMONIC DISTORTION

Distortion is related to INL

SFDR DEPENDENCE ON “N”

• Amplitude is • Frequency ~ f

• Amplitude is • Frequency ~ 2f

¼ the power over twice as many harmonicsPeak harmonic goes down by ¼ ½ = 9 dB SFDR ~ 9N dB

FLASH A-D CONVERSION

• (+) Parallel technique - low latency

• (+) References – resistor ladder

• (-) Complexity - O(2N)

• (-) Excessive power/area for N > 6

THE CLOCK SKEW ISSUE

• Sampling is distributed

• Problem : Clock skew causes different comparators to sample inputs at different instances

• Result : Poor performance at high input frequencies

• Solution : Make all comparator inputs see “held” inputs

PRACTICAL FLASH ADC

• T/H for good

dynamic

performance.

• Offset correction

in comparators.

BACKEND LOGIC

THE FOLDING ADC PRINCIPLE

• Motivation : Reduce latches & back end logic

WHITHER FLASH & FOLDING ?

• Disk drive read-channels• Low precision (6 bits)• Very high speed (Gbps)• Need very low latency (for timing recovery)

TWO STEP FLASH ADC

• Motivation: – Reduce number of comparators in a flash ADC

• Idea:– In a flash ADC, only comparators “near” the input give useful information– Use a coarse ADC to estimate where the signal is, then use a fine ADC placed “around” the coarse estimate for better accuracy

TWO STEP FLASH ADC

• Number of comparators : (2Nc + 2Nf – 2) • Resolution Nc + Nf bits• ADCs & DAC must be good to (Nc + Nf) bits

RESIDUE PLOT : A CLOSER LOOK

Slope = 1 ADC thresholds

Code : -2 -1 0 1

RESIDUE PLOT : A CLOSER LOOK

Code : -2 -1 0 1

DAC LSB

]DAC )5.0(Code [ V V LSBADCinr

]DAC )5.0(Code [ V V LSBADCrin

ISSUE : ADC THRESHOLD OFFSET

Fine ADC Range

Fine ADC overload !

ADC THRESHOLD OFFSET : FIX

• Extend the range of fine ADC by adding extra levels• Redundancy in fine ADC relaxes coarse ADC errors• Often called “Digital Error Correction”

ISSUE : DAC INACCURACY

)](DAC )5.0(Code [ V V LSBADCinr

Slope = 1 DAC level error

Fine ADC range not exercised

• DAC level too small : Missing codes• DAC level too large : Non-monotonicity

TWO-STEP FLASH SUMMARY

• Example : – 10-bit flash needs 1023 comparators– (5 + 5) bit 2-step flash needs only 62 comparators

• Lower hardware compared to a flash

• More latency (2 conversions, pipelined)

• Used for ~8-10 bit resolutions

• Coarse ADC resolution can be poor if extra levels are used in

the fine ADC

• DAC must be accurate to the resolution of the entire ADC

IMPROVED TWO-STEP FLASH ADC

• The fine ADC operates on a small input– Offset requirements for the fine ADC can be relaxed if the

input signal swing was larger– Amplify the input to the fine ADC

PIPELINE ADC PRINCIPLE

Recursive implementation of the fine ADC

PIPELINE ADC PRINCIPLE

• Nc bits per stage

• Issue with ADC threshold error

- Digital error correction • DAC & Interstage amplifier implemented with

switch-capacitor circuitry

STAGE 1 STAGE 2

OVERSAMPLING A-D CONVERSION

Oversampling ADC

)(2/

OSRRationgOversamplif

f

s

sig

Signal

Quantization Noise

Nyquist rate ADC

OVERSAMPLING & NOISE SHAPING IDEA

• Quantization noise is pushed out of the signal band• Digital filter required to eliminate out of band noise• Very high SQNR possible with a poor quantizer• Also referred to as ADCs

MODULATOR BLOCK DIAGRAM

• Discrete time input & output• Quantization modeled as additive noise

MODULATOR BLOCK DIAGRAM

Noise Transfer Fn. (NTF)Signal Transfer Fn. (STF)

)()(1

1)(

1)(

)()( zE

zLzV

zL

zLzV inout

Make L(z) large in the signal band

DELTA-SIGMA WAVEFORM EXAMPLE

Analog Input

Quantizer Output

OUTPUT SPECTRUM

Signal Bandwidth

Signal Tone

Shaped Quantization Noise

OTHER LOW SPEED ARCHITECTURES

• Algorithmic ADCs– Reuse a single stage of the pipeline

• Successive Approximation ADCs

• Dual-slope ADCs

HIGH SPEED : “SYSTEM” IDEAS

• Time – interleaving

multiple ADCs– N slow converters to

get one fast ADC– Sample and hold of

each converter must be good up to fs/2

– Gain & Offset mismatch– N times speed for N

times power

ADC FIGURE OF MERIT

• FOM Units : Joules per level resolved

• Small FOM means more power efficiency

• Higher speed – more power

• Higher resolution – more power

• ENOB : Effective number of bits for a Nyquist input

sfENOB2

Power FOM

FIGURE OF MERIT : COMMENTS

• For the same ADC spec, FOM usually improves with

technology (& skill of the designer !)

• Ponder this : If “X” can design a 10-bit 500 Msps ADC in a

certain process with 400 mW, does this mean that he/she can

design a 10-bit, 1Gsps ADC with 800 mW ?

sfENOB2

Power FOM

POWER EFFICIENCY OF ADCs

• Flash ADCs are least efficient– But unavoidable when latency cannot be

tolerated

• Pipelines more efficient than a flash– Reduced hardware at the expense of

latency

• Delta-Sigma more efficient than a pipeline– but only suitable for low bandwidth signals

REFERENCES

• M. Gustavsson, J. Wikner & N. Tan, “CMOS Data Converters for Communications”, Kluwer, 2000

• R. van de Plassche, “Integrated Analog-to-Digital & Digital-to-Analog Converters”, Kluwer, 1994

• R. Schreier, S. Norsworthy & G. Temes, “ Delta-Sigma Data Converters – Principles, Design & Applications”, Wiley, 1998

A-to-D Converters Specifications from

System Aspects

05/18/01 V4.3

ADC Parameters

• Sampling Frequency

• Resolution

• SFDR

• Input Bandwidth

• Latency

System Aspects…

• Signal Bandwidth

• System SNR requirements

• Signal characteristics– Dynamic Range– Interferers present– Single/Multi Carrier

Typical Real world signals

ADC requirements derived from Strengths of the Signal

and interferers.

-114dBm Noise

-104dBm

-14dBm

-40dBm

Blocker

Weak Signal

Sampling Frequency

Filters: DSL

30KHz 130KHz 1.1MHz

Upstream Downstream

FDM filters: Filters to filter-off the local-echo in a Frequency-Division-Multiplexed system

Filtering: Wireless signals

Filter to remove adjacent channel and other interferers.

-114dBm Noise

-104dBm

-40dBm

Weak Signal

Sampling Freq - Filtering

• Ideal anti-aliasing filter placed before an ADC – infinite attenuation for frequencies above

the cutoff frequency.

– sampling at fs=2fmax with no aliasing

• Very High order filters

– Typically > 8th order filters !!

High order filter

• High sensitivity, tougher to implement

• Process variation: requires calibration of filter

• High Power and Silicon area

• Non-linear phase response

• Effective solution is digital filters for anti-aliasing.

Receive channel

• Oversample the ADC– Simpler Lower order Analog filter– Anti aliasing done by the digital filter

• Filter –ADC complexity trade off

• ADC up in the signal chain– Signal processing in Digital domain

ADC

Digital filter Analog Filter ReceiverA/D Converter

Resolution

Resolution Specs

• SNR requirement

• Dynamic range of the input signal

• Signal characteristics:– Peak to Average (crest factor)

• Interferers present

ADC Requirement for DSL

SNR for QAM Modulation

(3dB/bit; 15 bits)

45dB

Crest Factor for clipping < 10-8 15dB

Dynamic Range lost due to Echo 0dB

Margin from ADC 10dB

70dB

System optimisation

Filter - ADC trade off

-114dBm Noise

-40dBm

Weak Signal

-90dBm

• Interferer higher power than the signal.

• For the signal to use the dynamic range of the ADC– Sufficient attenuation of the interferer ( < signal power)– AGC to gain the signal post filtering

• High order filter

-114dBm Noise

-40dBm

-90dBm

Filter - ADC trade off

-114dBm Noise

-40dBm

Weak Signal

-90dBm

• Trade off between filter order and dynamic range lost

in the ADC

-114dBm Noise

-40dBm

Weak Signal

-90dBm

SFDR

SFDR

• Caused by Harmonics and Intermodulation

products

SFDR

• Important parameter when digitizing

broadband multicarrier signals

• Desired signal then is obtained by using a

narrowband digital bandpass filter

• The SNR is improved by this digital-filtering

process – Advantage of oversampling

SFDR Performance of the ADC

• a spurious component may fall within the bandwidth of the digital filter

• SFDR: noise added in-band– does not improve with digital-filtering.

• SFDR spec > than the SNR after the digital filtering

• Example: 15b 65MSPS ADC with – SNR = 73dB– SFDR = 85dB

Frequency

Power

Spectrum

Missing Tone Test

180KHz 1104KHz

SFDR in a multi-tone system (DSL)

Input Bandwidth

• Critical parameter for systems that use

undersampling.

• Direct sampling of the IF signal in a wireless

system– Eg: 14b 125MSPS ADC sampling a

200MHz IF

• Jitter performance of the sampling clock for

this high input frequency..

Latency

• systems that use the ADC in control loops

• In communications– ADC used for AGC, power control

ADC: Bits vs. Speed Chart

5 10 15 20

1MHz

10MHz

100MHz

500MHz

1GHz

AD

C S

pee

d

Resolution (Bits)

Flash/Folding

Pipeline

Sigma-Delta

Basestations, Graphics

WLAN, DSL

Bluetooth

UWB, Disk Drives, Instrumentation

Pipelined A/D Converters

PIPELINE ADC PRINCIPLE - RECAP

Advances in ADC Speed

ADC Speed for 14-bit Resolution

100

150

200

2002 2003 2005

Year

Sp

eed

(M

SP

S)

• ADC speed increase driven by desire to bring ADC closer to antenna

Advances in Power Dissipation

ADC Power for 14-bit 105MSPS ADC

0

500

1000

1500

2000

2002 2004 2005

Year

Po

wer

(m

W)

• ADC power reduction driven by portability, SOC integration

• Near 2X reduction every 18 months

Continuous-time A/D Converters

C-T ADC: BASICS

• Architecture trades off speed for resolution; Ideal for applications where signal bandwidth is relatively small (example: Bluetooth, DSL, DVB-H)

• Signal sampled by the internal ADC free anti-alias filtering by the loop filter

• Important limitations are SNR degradation due to clock jitter (DAC) and stability concerns in higher-order loops due to excess delay (loop filter + internal quantizer)

ADCLoop Filter

H(s)

DAC

gm

vin bits

Advances in Power Dissipation

ADC Power for 12-bit ADC

0

50

100

150

2003 2005

Year

Po

wer

(m

W)

BW=40MHz; 74dB SNR

BW=10MHz; 67dB SNR

• Significant advances in the last two years; 4X improvement in speed, 5X reduction in power dissipation