Mixed-Signal Group

Murmann

Mixed-Signal Group

Murmann

Digitizing the Analog World:

Challenges and Opportunities

April 5, 2010

Boris Murmann

murmann@stanford.edu

2

Murmann Mixed-Signal Group

Research Overview

Signal

Processing

A/D

D/A

Signal

Conditioning

Signal

Conditioning

Transducers,

Antennas,

Cables, ...

Digital enhancement

algorithms

High-performance and low-

power A/D and D/A

converters

Sensor

interfaces

MEMS

Spin-Valve

Biomolecule

detection

Neural

prostheticsMedical ultrasound

3

Research Examples

High-performance A/D converters

Neural prosthetics

MEMS accelerometers

Large area electronics

4

Digitally Assisted A/D Converters

Signal

Processing

A/D

D/A

Signal

Conditioning

Signal

Conditioning

Analog Media

and

Transducers

DigitalAnalog

CLK

Additional digital processing for

performance enhancement

5

ADC for a “Digital” Serial Link

No analog error accumulation and better scalability

Need efficient high-speed ADC, typically > 10GS/s

6

ADC1

ADC2

ADCN

text

X(t) Y[n]

Time-Interleaving

1

2

N

7

Popular way to increase ADC throuhgput

ADC1

ADC2

ADCN

X(t) Y[n]

G1

Voff_2

G2

Voff_N

GN

Voff_1

text

Imperfections

1

2

N

8

Mismatches result in signal distortion Gain

Offset

Timing Skew

Our Focus: Timing Skew

9

1

2

(2-channel example)

Digital Backend

ADC1

ADC2

ADCN

X(t)

ADCCal

Y[n]

Clock

Skew Calibration Using Extra ADC

Statistics-based skew measurement in digital backend

Correction through analog adjustments

10

1

2

N

Cal

1 2

Digitally

adjustable

delay cells

N

Timing of Auxiliary ADC Phase

1

2

N

Cal

1

2

N

Cal

1 2

Digital Backend

ADC1

ADC2

ADCN

X(t)

ADCCal

Y[n]

Clock

11

1

2

N

Cal

Calibration Scheme

For each channel, adjust delay cells until correlation between

calibration ADC output and each slice are maximized

ADCCal can be 1-bit and “slow”

12

ADC1

ADC2

ADCN

X(t)

ADCCal

Y[n]

ClockMax

1

2

N

Cal

1 2

R()

Removed pre-publication slides on experimental

results…

13

14

MEMS Accelerometer

Capacitance change ~10 fF/g

Desired resolution ~10 mg for airbags and ESP Must resolve capacitance changes of ~100 aF

Problem: Drift in parasitic bondwire capacitance

CMOS

15

Sigma-Delta Interface

INam

kbsms 2

1 x

xC

C

VCA V

S/HLead

Compensator

DigVmechF

Force-

Balancing

DecimatorOutV

M. Lemkin and B. E. Boser, “A three-axis micromachined accelerometer with a CMOS

position-sense interface and digital offset-trim electronics,“ IEEE J. Solid-State Circuits,

vol. 34, pp. 456-468, April 1999.

Mechanical

16

Offset

INam

kbsms 2

1 xx

C

CVCA

VS/H

Lead

Compensator

mechF

Force-

Balancing

OffsetC

Offset due to bond

wire deformation

17

Linear Feedback System with Two Inputs

1 2

1 1y x x

f af

ayx1

f

+ _ b+

x2

18

Spring Constant Modulation

The output due to Coff can be modulated to higher

frequencies by modulating the spring constant k

1Out mech Off

k kV F C

CFBFB

x

INam

kbsms 2

1 xx

C

CVCA

VS/H

Lead

Compensator

mechF

Force-

Balancing

OffsetC

19

Spring softening effect

Acceleration

Spring

Acceleration

Spring

_ _+

_ _+

_ _+

_ _+

Electrostatic

Can be used to modulate spring constant (k)

20

Modulation through Multiplexed Feedback

Time-Multiplexed

INam

kbsms 2

1 x

xC

CVCA

VS/H

PULSE

OutVmechF

Electrostatic

Force

Decimator

xkm kf

Int Com

T T

MOD MOD Force-BalancingForce-Balancing

Output Spectrum with 1-Tone Modulation

21

10-6

10-5

10-4

10-3

10-2

10-1

-160

-140

-120

-100

-80

-60

-40

-20

0

Frequency (MHz)

Po

we

r/fr

eq

ue

ncy (

dB

/Hz)

10-6

10-5

10-4

10-3

10-2

10-1

-160

-140

-120

-100

-80

-60

-40

-20

0

Frequency (MHz)

Po

we

r/fr

eq

ue

ncy (

dB

/Hz)

10-6

10-5

10-4

10-3

10-2

10-1

-160

-140

-120

-100

-80

-60

-40

-20

0

Frequency (MHz)

Po

we

r/fr

eq

ue

ncy (

dB

/Hz)

-89 dB

-32 dB

-46 dBDC

Acceleration

Offset

Capacitance

0 fF

10 fF

50 fF

9.1 m/s^2

9.1 m/s^2

9.1 m/s^2

100

102

104

106

-140

-120

-100

-80

-60

-40

-20

Frequency [Hz]

Outp

ut S

pectr

um

[dB

]

Modulating spring-constant with a pseudo-random sequence

22

Pseudo-Random Modulation

23

Parameter Convergence

0 0.5 1 1.5 2-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Time [Sec]

Feedback s

ignal [x

10

-15]

Closed-loop system - Feeding back capacitance

Coff=0fF

Coff=0.01fF

Coff=0.1fF

Coff=1fF

Chip Design in Progress

24

CMOS

MEMS

C to V Integrator Compensator Quantizer

Electrostatic

Feedback

State-

Machine

VOut

Clk

VRef Gnd

FPGA

DecimatorCorrelator

DOut

k-modulation

Scan In/Out

Neural Prosthetics

Cortical motor prosthetics Neurons in the motor cortical areas of the brain encode

information about intended movement

25Courtesy K.V. Shenoy Courtesy L.R. Hochberg

Nature Magazine June ‘06

Neural Signal Acquisition

Electrode signals consist of multiple sources DC Offset, about 15mV from electrode/tissue interface

Local field potential (LFP), ≤3mV peak, 10Hz to 100Hz

Spikes from nearby neurons, 35μV – 1mV peak, 500Hz to 5kHz

26

Courtesy C.L. KlaverCourtesy M. Sahani

Specs

27

Spikes Local Field Potential

Gain 600 V/V 200 V/V

Lower Cutoff 300Hz 1Hz

Upper Cutoff 10kHz 1kHz

Input Referred Noise

(total from sampling node)2.0µVrms 1.0µVrms in 10-100Hz

Total Power (96x Array) 3mW 100µW

Separate the fast and slow signal acquisition for DR Custom front end design for each path

Spike Path Front-End

28

Input Stage

SC

Bandpass

Filter

SAR ADC

Input Cap

Output

Buffers

Sampling Phase

29

Integrate signal current on

CB and sample

High-pass for DC block

using Cac and Rbig (off-

resistance)

A1 contains a pole that

helps minimize noise

folding

A1 Implementation Details

30

ITAIL I<< ITAIL

Voutp

Voutm

VB1

VB2

M1a M1b

Flicker noise

reduction

Anti-alias for

thermal noise

from M1a,b

Static Power

31

Two-Channel Interface Pixel

32

Frontend

SAR ADC

Die Photo (96 channels, 5mm x 5mm)

33

The Future?

34

Organic Semiconductors

Mechanically flexible

Suitable for solution processing Cover large areas at low cost

Make disposable devices

35

Jellyfish Autonomous Node

36

http://muri.mse.vt.edu/

Jellyfish Bell Prototype (Virginia Tech)

37

A bio-inspired shape memory alloy composite (BISMAC) actuator

A .A .Villanueva, et al., 2010 Smart Mater. Struct. 19 025013 (17pp)

Want to Make Plastic ADCs !

38

6-bit A/D Converter Prototype

39

W. Xiong, U. Zschieschang, H. Klauk, and B.

Murmann, “A 3V, 6b Successive Approximation ADC

using Complementary Organic Thin-Film Transistors

on Glass,” ISSCC 2010.

Substrate Glass

Interconnect Ti/Au evaporation, litho, wet etch

Gate electrodes Al evaporation, shadow masking

Source/Drain Au Evaporation, shadow masking

Dielectric 5.7nm AlOx/SAM

PFET DNTT, ~0.5 cm2/Vs

NFET F16CuPc, ~0.02 cm2/Vs

Area 28mm x 22mm

Component count 74

ADC Schematic

40

VREFN

C/32 C/32C/32

...

...

Bit 0 Bit 1 Bit 5Bit 2, 3, 4

Input

SAR Logic

(off-chip)

Output

To DAC

ComparatorDAC with

Sampler

C CCC 2C 2C

VREFN VREFNVREFP VREFNVREFP VREFP VMID

VREFPVMID

VREFN

Calibration DAC

Main DAC

Calibration enables 6-bit

precision despite poorly

matched capacitors

C-2C structure

possible due

small stray

caps (glass)

Comparator

41

CS1 CS2 CS3 CS4

CLK

CLK

CLK

CLK

CLK

CLK

CLK

CLK

Wp = 500um

Wn = 500um

Lp = Ln = 20um

CS1 = 1.1nF

A0 = -7.8

= 3.0ms

Wp = 400um

Wn = 400um

Lp = Ln = 20um

CS2 = 1.1nF

A0 = -9.8

= 4.0ms

Wp = 400um

Wn = 400um

Lp = Ln = 20um

CS3 = 1.1nF

A0 = -9.8

= 4.0ms

Wp = 100um

Wn = 300um

Lp = Ln = 20um

CS4 = 1.1nF

A0 = -19.6

= 16.4ms

Transmission Gates:

Wp = Wn = 100um

Lp = Ln = 20um

CF1 CF2 CF3 CF4

+ -

Input Output

Auto-zeroing

cancels threshold

voltage drift

Anti-parallel PFET/NFET

layout minimizes variations

if CF due to misalignment

CgdnCgdp

0 8 16 24 32 40 48 56 63-4

-2

0

2

4D

NL

(L

SB

)

Code

0 8 16 24 32 40 48 56 63-4

-2

0

2

4

Code

INL

(L

SB

)

Measured DNL/INL

42

Before calibration, 100 Hz clock rate

0 8 16 24 32 40 48 56 63-1

-0.5

0

0.5

1D

NL

(L

SB

)

Code

0 8 16 24 32 40 48 56 63-1

-0.5

0

0.5

1

Code

INL

(L

SB

)

Measured DNL/INL

43

After calibration, 100 Hz clock rate

Organic ADC Summary

44

Process3 metal complementary

organic thin-film

Minimum feature size 20 mm

Chip area 28 mm x 22 mm

Resolution 6 bits

Full-scale range 2 V

Max DNL / INL -0.6 LSB / 0.6 LSB

Clock rate / Update rate 100 Hz / 16.7 Hz

Power consumption 3.6 mW @ 3 V

45

Conclusions

Mixed-signal IC design remains a vibrant area of

research

Changing boundary conditions Ever-increasing need for higher performance, lower power

New applications

New device technologies

A recurring theme in our research Looking for new ways to overcome analog imperfections

using DSP and calibration