May 2, 2012 1

Practical issues and solutions in

Ultra Low Power designfor Artificial Retina

May 2, 2012

Tuvia LiranNano Retina Inc.

May 2, 2012 2

Outline

• What is Artificial Retina• Key technical challenges• Micro-architecture of Artificial Retina• Challenge #1 – power reduction• Challenge #2 – selection of supply source• Challenge #3 – controlling very low bandwidth• Summary

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Vision Pathway with Bio-Retina

Optic nerve

Macula

Retina

Bio-Retina glass

Bio-Retina Implant

Ganglion Bipolar Photoreceptors

Nano Retina Confidential3

What is artificial retina

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• Bio-Retina implant includes:

– Receives visual images

– Converts the image into neuron stimulation

– Operates from infrared radiation power source

– Configured by wireless optical transmission

• Eyeglasses includes:

– Infrared power source

– Rechargeable battery

– Implant control

Artificial Retina technology byNano Retina

IR laser source

Bio-Retina Implant Retina

IR laser beam

Advantages: Light and long lasting Simple implantation Uses the eye’s natural optics

and nerves

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Nano Retina technology

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Key technical challenges • Ultra low power (ULP) (<300uW)

• Weak power source• Very low bandwidth (1÷40Hz)

• Wide dynamic range (>5 decades = ~17 bits)

• Sufficient resolution (~1000 pixels)

• Wirelessly configurable (w/o antenna)

• Very small device (<15mm2 including package)

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• Phase 1: Providing proof of concept– Development of Micro-electrode array

(MEA)– Defining process flow & vendors– Development of ULP test chip– Performing pre-clinical experiments

• Phase 2: Development of prototypes– Fully compliance for medical experiments– Full functionality– Complementary accessories

Key development steps

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Micro architecture of Artificial Retina

• MEA: by NR• Design: CSEM

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• Process: TSL018 CIS• Power: GaAs PV

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Challenge #1: Very low power• Why it is important?

– Ability to transfer optical power without exceeding eye safety limits– Minimizing heating of the eye– Minimizing the batteries on the glasses

• Considering the use of regulator– Regulator consume significant portion of total power– Regulator reduces the usable voltage swing– Regulator is a must when AC power source is AC or unstable– Regulator reduces the performance variation due to supply voltage

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How it is achieved?• ULP oriented micro-architecture

– Minimum usable and achievable voltage– Minimum digital processing– Minimum frequency at any stage– Discrete time analog processing

• ULP analog design style by ULP experts• Low voltage operation

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ULP design techniques are enablers for implementing medical implantable devices,

such as Artificial Retina

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Challenge #1a – sub-threshold circuits

• Vdd ~Vth -> operation at sub-threshold region• Selecting proper circuits:

– Dynamic range– Power– Linearity– Matching– Noise

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Operating CMOS in sub-threshold

• Operating in sub-threshold (weak inversion)

• Voltages are scaled to nUT (32 mV for n = 1.2 )

• ∆Id/∆Vgs = 70÷80mV/decade • Weak inversion expression:

T

TGsDsat nU

VVII 0exp

C37at mV 7.26 Tq

kTUT

CSEM Microelectronics

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Example of sub-threshold low voltage circuit – ULP amplifier

Presented by E. Vittoz

Amplifier with controlled offset

Amplifier with extended

dynamic range

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Challenge #1b – device modeling

• Conventional BSIM3/4 are not accurate at sub-threshold

• EKV models are more accurate (availability ?!)• Need also accurate models for monte-carlo & noise

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Challenge #1c – mismatch in sub-threshold circuits

• Mis-matching is high• Mitigation techniques:

– Large devices– Very careful layout – matching rules– Dynamic offset cancellation (chopping):

Presented by C. Enz & G. Themes

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Challenge #2 – Selection of power source

• Relevant power sources:– Battery– Energy harvesting– Electromagnetic power– Optical power transfer

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Performance limitations of PV

• Limited selection of voltages• Limited current• High dV/dI• Fast voltage drop at over-

loading

Laser IR transmitter + Photo-Voltaic receiver is the preferred power source for Artificial Retina. But…

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Specifying voltage requirements

• Analog circuit requires higher supply voltage than digital

• Limited by dynamic range of analog circuits– Example: Logarithmic Trans-Impedance Amplifier

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Challenge #3 – controlling very low bandwidth

• Bandwidth of neural signals is <<100Hz– RC example: C << 10pF ; R >> 10Gohm (!!!)

• Analog implementation of high giga-resistor:– Transistor at very weak inversion– Linearity ?!– Immunity to leakage ?!– Matching ?!

• Switch capacitor implementation:– CMOS switches– Charge injection !!!

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Implementing ULP switch capacitor resistor

• General SC resistor– Minimum size CMOS switches– Capacitors implemented by:

• NMOS• MIM• Native_NMOS

• Charge injection:– Might cause offset– Difficult to predict– Difficult to match

• Differential SC circuits are better

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Summary• ULP is enabling technology for implantable medical devices,

such as Artificial Retina• ULP implementation is challenging but doable• Key factors for successful design:

– Optimal micro-architecture– Optimal selection of power sourcing– Know-how in sub-threshold design– Availability of EKV models– Intensive monte-carlo simulations