Download - SAR+ ADC with Open-Loop Integrator using Dynamic … SAR+ADC with Open-Loop Integrator using Dynamic Amplifier Akira Matsuzawa and Masaya Miyahara Tokyo Institute of Technology Contents

2017.10.26

SAR+ADC with Open-Loop Integrator using Dynamic Amplifier

Akira Matsuzawa and Masaya Miyahara

Tokyo Institute of Technology

1Contents

Background and Motivation Design concepts

• ADC architecture• Open-loop integrator• Dynamic amplifier• Binary-mode DEM

Measurement resultsSAR+DS ADC for CMOS image sensorConclusion

2017.10.26 A. Matsuzawa, Tokyo Tech.

2Background SAR ADCs Lowest power consumption with clock scalability SNR is limited by noises (Comp., Ref., mismatch)

∆Σ ADCs Highest SNR Power consuming of an Opamp in an integrator

Noise-shaping SAR ADCs [1, 2] High SNR and high efficiency Opamps are still used

[1] P. Harpe, et al., ISSCC 2014, [2] Y-S Shu, et al., ISSCC 2016


3

biasperiod conversion ADCtime OperatingtimeRecovery current average Opamp I

Previous works 1

[3] K. Obata, et al., VLSIC 2016

Switched Opamp [3] Overhead of the recovery time


4Previous works 2Fully passive integrator [4] Low gain, incomplete integration => Only for low OSR

[4] Z. Chen, et al., A-SSCC 2016

S

NS1

NS2

s

1 2

NS1

NS2

3

top

2

2 1

1

3 1

1 1,

1

0.5 1 1 0.6 ( )1 0.2 0.5

res plusz z V zY X

z


5Previous works 3Passive integrator with dynamic pre-amp [5] Higher gain, but incomplete integration

[5] C-C. Liu, et al., ISSCC 2017


6Effect of incomplete integration

An incomplete integrator cannot suppress Qn at low freq.

-80

-60

-40

-20

0

20

0.001 0.01 0.1

Qn

PSD

(dB

)

Normalized frequency

Ref. [5]Ref. [6]Idel 2nd order

Ex.) @ Fin = 1/20Ref. [5] = -15.0 dBRef. [6] = -21.3 dBIdeal = -32.2 dB


[4][5]

[4][5]

7Our Design Concepts

Open-loop Integrator • A complete integrator can be realized by

using a low gain open loop amplifier and switched capacitors

• Dynamic amplifiers achieve low power consumption and clock scalability

Binary-mode DEM• Simple structure• Low power and small area


8Target SQNR

Target SQNR > 95dBSAR ADC part : 6bit, SQNR = 38dBDSM part : Qn suppression > 57dB @OSR=20

0

20

40

60

80

100

120

1 10 100

SQNR

(dB

)

OSR

2nd order3rd order


9ADC Architecture

Reset Sampling SAR Conversion Integration


10ADC Architecture



11Proposed Open-loop Integrator

Phase: 1Vout = Vout_n-1

Phase: 1Vout = Vout_n-1V1 = A1VinV2 = A2Vout_n-1


12Proposed Open-loop Integrator

Phase: 1Vout = Vout_n-1

Phase: 2Vout = Vout_n-1+Vin

Phase: 1Vout = Vout_n-1V1 = A1VinV2 = A2Vout_n-1

Phase: 2Vout = (Vout_n-1+V1+V2)/3A1=3, A2=2,

Vout = Vout_n-1+Vin


Proposed method can realize a complete integration

13Dynamic Amplifier Design• No DC current• Gain controllable 1.5x～4.5x, 5bit resolution


14Open loop vs. Closed loop

An open loop integrator achieves 50~90% power reduction

0200400600800

100012001400160018002000

0 1 2 3 4 5 6 7 8 9 10

Aver

age

curr

ent (μA

)@10

MS/

s

Opamp recovery time (ns)

90%

50%Proposed

TelescopicOpamp

Folded cascodeOpamp

Proposed OpampDC Gain(V/V) 3 100# of unit 2 1IntegratorType Open Closed

IntegratorOutput noise 100μV RMS

Settling error - 1%Settling time 1.8nsClock Freq. 150MHzRecoverytime None 1 CLK

(6.7ns)


15Gain error influence

80

85

90

95

100

105

110

-100 -50 0 50 100

SNDR

(dB)

Gain error (%)

Simulated SNDR vs. gain error of amplifier used in 1st integrator

Gain error of ±20% can be accepted (|∆SNDR| < 3dB)

3dB down

%20


16ADC Architecture



17Proposed Binary-mode DEM (B-DEM)

Randomly select left(0) or right(1) unit capacitors in each conversion.

DEM Control Code(DCC)

1XX

11X

10X

111

110

101

100

0XX

00X

01X

000

001

010

011

MSB conversion

MSB-1 conversion

LSB conversion

3bit example


18Effect of proposed B-DEM

-140

-120

-100

-80

-60

-40

-20

0

1 10 100 1000 10000

Nor

mal

ized

Pow

er [d

B]

Frequency [kHz]

-140

-120

-100

-80

-60

-40

-20

0

1 10 100 1000 10000

Nor

mal

ized

Pow

er [d

B]

Frequency [kHz]

B-DEM disable

B-DEM enable

OSR=16, Mismatch 0.1%20 times Monte Carlo Simulation

SFDR (dB) SNDR (dB)Disable 88.2 83.0Enable 103.2 88.5

Improvement +15.0 +5.5


19Chip photo• 65nm 9M1P CMOS technology • Chip area of 0.08mm2


20FFT Spectrum

-140

-120

-100

-80

-60

-40

-20

0

1 10 100 1000 10000

No

rm

ali

zed

Po

wer [

dB

]

Frequency [kHz]

Without DEM

With DEM

Fs=10MS/s, Bandwidth = 250kHz, OSR=20, 10kHz input

BW=250kHz

SNR=84.2dBSFDR= 96.5dBSNDR=83.4dB

SNR=84.2 dBSFDR=96.5 dBSNDR=83.4dB

BW=250kHz

Without DEM

Without DEM

Fs=10MS/s, BW=250kHz, OSR=20, 10kHz input

Frequency [kHz]

1 10 100 1000 10000

0

-20

-40

-60

-80

-100

-110-120

-140

Nor

mal

ized

Pow

er [d

B]


21Dynamic range

Dynamic range = 84.2 dB

-20

0

20

40

60

80

100

-100 -80 -60 -40 -20 0 20

SNR

,SN

DR

[dB

]

Input power [dBFS](Fs=10MS/s, OSR=20)

SNRSNDR

Dynamic range = 84.2dB

808182838485

-3 -2 -1 0 1


22Clock scalability

FoMs > 170 dB for different sampling rates of 2.5 - 25 MS/s.

Pc @ 10MS/sAnalog : 60.0W (23.3%)Ref : 35.1W (13.6%)Digital : 162.7W (63.1%)

70

90

110

130

150

170

190

0

200

400

600

800

1000

1200

0 5 10 15 20 25 30

FoM

, SFD

R, S

ND

R [d

B]

Pow

er c

onsu

mpt

ion

[mW

]

Sampling rate [MS/s](OSR=20)

FoM

SNDR

Power

SFDR


23Performance Summary

This Work VLSI 2016 [3]

ISSCC 2016 [1]

ISSCC2014 [2]

Architecture NS SAR NS SAR NS SAR NS SARProcess [nm] 65 28 55 65

Active Area [mm2] 0.08 0.116 0.072 0.18Supply [V] 1.0 1.55/0.75 1.8/1.1 1.2 0.8

Sampling rate [MS/s] 2.5 10 25 0.1 1.0 1.0 0.128BW [kHz] 62.5 250 625 2 20 4 16

SFDR [dB] 88.3 96.5 89.9 111.8 108.0 105.1 86.9SNR [dB] 84.3 84.2 82.2 98.57 94.44 96.8 -

SNDR [dB] 82.3 83.4 80.4 97.99 93.95 96.1 76.1Power [W] 66.3 257.8 630.2 37.1 493.1 15.7 1.37

FoM [dB] 172.0 173.3 170.4 175.3 170.0 180.0 176.8

Recent noise shaping SAR ADC


24Performance comparison

10

100

1000

10 100 1000 10000

Wal

den

FoM

[fJ

/con

v.]

Bandwidth [kHz]

This work

140.0

150.0

160.0

170.0

180.0

190.0

10 100 1000 10000

Schr

eier

FoM

[dB

]

Bandwidth [kHz]

This work

Compared with the state-of-the-art ADCs over 80 dB SNDR


25SAR+ ADC for CMOS image sensor

We have applied our proposed ADC technique to CMOS image sensor for reducing the read noise and power reduction.


A. Matsuzawa and M. Miyahara, IISW, May. 2017.

26

∫ ∫

Proposed ADC for CIS

A. Matsuzawa, Tokyo Tech.

CDAC

6b+shift

CDAC

Int. Int.

RST

DynamicComp.

Vin

Vref

Trig

LogicsControl

Oscillator

Dout

2

for SAR and S/H

Low power SAR ADC + low noise ADC

1b+overlap

2017.10.26

2nd order incremental SAR+ADC

27 ADC with multiple sampling


Vin Vrefa1 a2

b

b

a1

a2

C C1

C1

C

Repeat

Vres

Signal sampling Residue generation

S/H 1 2 6 1 2 M

SAR ADC; 6 times ADC; m=32, 64, 128

Multi-samplingTrigger RST

After the SAR conversion, ADC is performed.The signal is sampled and the residue voltage is generated in CDAC using the converted data.

8-10 ns

2017.10.26

28CDS and ADC


1

01

0

Vr(6) Vq_DS

VRST

Vin

Vs

LSB

Shift

△ΣADCOverlap

Correlated Double Sampling (CDS)

SAR ADC

Vq_DS=30mV

△ΣADC is performed in small Vq of 30 mV with overlapping.The effect of capacitor mismatch is avoided in CDS operationby fixing the CDAC condition for the small signal.

fix the CDAC condition

2017.10.26

29Layout of the ADC20μm

770μm

CDAC COMP LOGIC VCO 1st Integrator 2nd Integrator


The ADC is designed in 65 nm CMOS technology

CDAC COMP LOGIC VCO

1st Integrator 2nd Integrator

2017.10.26

30Measured noise voltage


MeasuredADC for CIS

SimulatedMeasuredGeneral purpose ADC [2]

[2] M. Miyahara, et al, CICC, April, 2017

Measured noise is quite larger than that of the simulated. Our previous SAR+ADC realized much lower noise.

66 V

24 V

Oversampling ratio, m

Noi

se v

olta

ge (μ

Vrm

s)

m=16

m=32m=64

m=128

1000

100

1010 100 1000

2017.10.26

31What is the difference ?


General purpose [2]

This work for CIS

General purpose [2] This work for CISSignal: Samplig Full differential Single endedSignal: Integrator Full differential Full differentialCapacitance (pF): Sampling 10 1Capacitance (pF): Integrator 5 0.5Signal swing 2 Vpp 1 VQuantization voltage (mV) 16 32Order of integrator 3 2 (Incremental)

2017.10.26

32Conclusion

• Open-loop integrator A low gain open-loop amplifier can be used. No static current, clock scalable by using dynamic amplifiers. 50~90% power reduction compared with conventional

closed-loop integrator.

• Binary-mode DEM Simple shuffler +15dB of SFDR @ OSR=16

• An 84 dB dynamic range, 62.5-625 kHz bandwidth, FoMs > 170 dB noise shaping SAR ADC is realized.

• SAR+ADC is developed for low noise, low power, and high speed ADC for CMOS image sensor.
