practical solutions in ultra low power design for artificial retina
DESCRIPTION
Tuvia Liran,Nano RetinaTRANSCRIPT
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May 2, 2012 1
Practical issues and solutions in
Ultra Low Power designfor Artificial Retina
May 2, 2012
Tuvia LiranNano Retina Inc.
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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