Constandinou, TG

Georgiou, J

Toumazou, C

Spiral - Imperial College Digital Repository

A Fully-Integrated Semicircular Canal Processorfor an Implantable Vestibular ProsthesisTimothy G. Constandinou∗†, Julius Georgiou∗, and Christofer Toumazou†∗Holistic Electronics Research Lab, ECE Department, University of Cyprus, Nicosia 1678, Cyprus†Institute of Biomedical Engineering, Imperial College of Science, Tech. & Medicine, London SW7 2AZ, UKEmail: t.constandinou@{ucy.ac.cy,imperial.ac.uk}, julio@ucy.ac.cy, c.toumazou@imperial.ac.ukAbstract—This paper presents a fully-integrated semicircularcanal processor for an implantable vestibular prosthesis. Thesystem includes the entire processing chain required from themicrosensor interface, through the required biomimetic transfercharacteristics, to a current-mode signal, suitable for feedingthe stimulator. Using a switched-capacitor implementation, thecircuit is inherently tunable, whilst maintaining low poweroperation and overcoming the need for off-chip components. Theentire system core occupies an area of 0.23mm2 in a commerciallyavailable 0.35µm CMOS technology.Index Terms—semicircular canal, vestibular prosthesis,switched-capacitor filter, analogue signal processingI. INTRODUCTIONRestoring the sensation of inertia in individuals withbalance-related impairments is achievable through develop-ment of an artificial vestibular prosthesis, based on thecochlear implant paradigm. The inner ear’s vestibular systemprovides cues about self-motion and help stabilise vision dur-ing movement. Damage to this system can result in dizziness,imbalance, blurred vision and instability in locomotion, a lead-ing cause of death in the elderly. Restoration of balance cantherefore be achieved by bypassing a dysfunctional element inthe vestibular pathway using artificial stimulation.A vestibular prosthesis would incorporate miniature(MEMS-based) inertia sensors, suitable sensor interfacingcircuits [1], a semicircular canal processor and stimulationdriver [2]. In the implementation, only the semicircular canalprocessor requires a custom design specific to the vestibularprosthesis. In comparison, a recent trend that has emerged incochlear prostheses, is the aim to implement the entire systemon a single chip, including custom analogue signal processors[3]. Research in developing artificial vestibular prostheseshas also gained much momentum, including several groupsimplementing the semicircular canal dynamics in differentways. For instance, Ciavella et al. [4] have developed abio-mechanical model of the vestibular system to supportrelated prosthesis research. Shkel et al. [5] have implementeda discrete semicircular canal processor whereas Gong et al.[6] and Santina et al. [7] have chosen to implement a micro-controller based processor.This work [1], [2] aims to implement a custom hardwaresolution to achieve a single chip semicircular canal processor.Methodologies to implement custom transfer functions inintegrated analogue hardware, include current-mode process-ing (log-domain and Gm-C) [8] and sampled-data techniques(switched-capacitor [9] and switched-current [10]). A majorchallenge when implementing such biomimetic algorithms inanalogue hardware, typically is the requirement for functionswith long-time constants. This inherently demands large ca-pacitors that does not allow for compact implementation. Toovercome this, we have chosen to use a switched-capacitortechnique as transfer function time-constants are directly pro-portional to the clock frequency. This paper therefore de-scribes a fully-integrated switched capacitor implementation toreplicate the semicircular canal dynamics of the physiologicalvestibular organ. The paper is organised as follows: Section IIdescribes the vestibular physiology and target transfer dynam-ics, Section III describes the system architecture and circuitimplementation, Section IV presents simulated results, andfinally Section V provides a short conclusion.II. THE PHYSIOLOGICAL SEMICIRCULAR CANALThe inner ear constitutes two sub-organs, the cochlear andthe vestibule, responsible for transducing sound and inertiato the vestibulocochlear (VIII) nerve respectively. In turn, thevestibular system contains three semicircular canals in eachear, approximately orthogonal to each other to sense motionin the three axes. The semicircular canals are fluid filledmembranes that form about two-thirds of a circle with oneend feeding from a reservoir of fluid and the other terminatingat a bulb like structure, the ampulla. Inside this exists amembrane containing blood vessels and nerve fibres reachingthe sensory receptor area. Hair cells are located on the surfacewith their cilia (hair like structures) protruding into the centreof the ’bulb’ volume. As the head moves, the fluid in eachsemicircular canal, is free to move along the cylindrical cavity.This in turn exerts a force via the bulb-like membrane; whichdeflects in the direction of the force essentially acting as acoupler, transmitting the mechanical energy to the hair cells,and thus leading to the production of action potentials.In all mammals, rotational velocity over a relatively broadrange of frequencies is encoded by the regular neurons in-nervating the semicircular canals. At intermediate frequencies(between 0.05Hz and 5 Hz) the semicircular canals act like ve-locity sensors. At lower frequencies, with a cut-off frequencybetween 0.025 and 0.05 Hz (corresponding to a time constantbetween approximately 6 and 3 seconds, respectively), thehigh-pass filter characteristics of the canals become evident.978-1-4244-2182-4/08/$25.00 ©2008 IEEE. 81IOUTV to IConverterLow-PassFilterSample& HoldHigh-PassFilterAll-PassFilter & GainCapacitor toVoltageNon-overlappingClock phase GenerationIBIASH1(s) H2(s) H3(s) F2(V)F1(C)VOA  φ1 φ1 φ2 φ2 VOA  φ1 φ1 φ2 φ2 VOA  φ1 φ1 φ2 φ2 VOA  φ1 φ1 φ2 φ2 VOA  φ1 φ1                        φ1 φ1 φ2 φ2VOA                 IOUTVIN                                VOUTVOUT VIN                                VOUT VIN                                VOUT VIN                                VOUT VIN CLKFCLKC-            COM            C+Switched Capacitor Transfer FunctionFig. 1. Top level system architecture of the semicircular canal processor. This interfaces to a capacitive microsensor to produce a proportional voltage. Thisis fed through the switched-capacitor filter which implements the biomimetic transfer function. The output is then converted to a suitable reference current todrive the electrical stimulation driver.The afferent neurons have a resting discharge which variesfrom a few to over 200 spikes/s.These biomechanics can be described by a damped oscilla-tor [11], i.e. based on a pendulum model. If the deflectionof the cupula is defined with the angle θ, and the headvelocity with q̇, the cupula deflection can be shown to beapproximately:θ(s) =αs(τ1s+ 1)(τ2s+ 1)q̇(s) (1)Where α is a proportionality factor, s corresponds to the fre-quency and the time constants τ1 and τ2 define the pendulumdynamics.Furthermore, by additionally considering the spike gener-ation dynamics to a linear approximation, the relationshipbetween input angular acceleration and change in (hair cell)output pulse rate is described by:H(s) =τAs1 + τAs· 1 + τLs(1 + τ1s)(1 + τ2s)(2)Where H(s) is the output pulse rate, τA represents the neuraladaptivity, τL is the dynamical-electrical time constant, and τ1and τ2 are defined as in Eqn. 1.From animal experiments (squirrel monkey) [12], the timeconstants τ1, τ2, τA and τL have been determined to beapproximately 3ms, 6s, 80s and 49ms respectively.III. SYSTEM ARCHITECTURE AND CIRCUITIMPLEMENTATIONThe top level system architecture of the system implementedis shown in Fig. 1. The subsections that follow describe thecircuit implementation of various blocks shown in the systemarchitecture.-+VOUTCFBφ1φ1φ2φ2φ2 φ1C0+½ΔCC0-½ΔC+VREF-VREF-VREF +VREFFig. 2. Circuit schematic of the capacitive microsensor interface cir-cuit based on a switched-capacitor charge integrator. Feedback capacitor ischosen to be 0.5C0. All switches implemented as transmission gates withW/L=1µm/0.5µm.A. Capacitive Microsensor InterfaceThe input stage of the semicircular canal processor inter-faces to a capacitive microsensor, to provide the inertia inputto the system. The circuit schematic for this sensor interfaceis shown in Fig. 2. This consists of a switched capacitorcharge integrator to measure a differential capacitance in therange 0-10pF, with 50fF target resolution. The circuit operatesas follows: During φ1, the output capacitor is reset anddifferential voltage applied across the input capacitors. Duringφ2, the differential voltage applied across input capacitors isinverted and the differential charge difference is convertedto a voltage across the op-amp feedback capacitor (CFB).The input/output characteristics (i.e. capacitance to voltagerelationship) is given by:Vout =Vref ·∆CCFB(3)82-+φ1φ1φ2φ2φ2φ1VINC2C1C5C4C3-+φ1φ2VAC7φ1φ2C6φ2-+VBφ1φ2C8φ1φ2φ1φ2C9C10VC-+VOUTφ1C11Gain Stage Low Pass Stage High Pass Stage Sample & HoldFig. 3. Circuit schematic of the switched capacitor filter for implementing the semicircular canal transfer function. This is implemented as a cascade offour sub-blocks, including high-pass, gain, low-pass and sample/hold stages (shown from left to right). The circuit requires two clock phases that are non-overlapping. Relative capacitances are as follows: C1=250, C2=1, C3=C4=2, C5=85, C6=40, C7=1, C8=C9=C10=5, C11=5. The unit capacitance, Cminis 100fF.B. Switched Capacitor FilterThe circuit schematic for the switched capacitor filter isshown in Fig. 3. This consists of a four op-amp cascade ofswitched capacitor filter stages.Firstly, a high-pass characteristic with the lower frequencyfeatures (including two zero’s and one pole) is implementedadopting the Nagaraj technique [13] to achieve a wide spreadin pole/zero frequencies, i.e. (ratio of Fclk to Fz and Fp). Thetransfer function for this stage is given by:H1(s) ≈ −C1C3· 1 + C1C3s1 +[1 + C4C5]C5C4C5C3s(4)The second stage then implements a basic switched capac-itor (inverting) gain stage with transfer function: H2(s) =−C6/C7 and third stage realises a low pass (single pole)operation with transfer function:H3(s) = −C8C9· 11 + C10C9·fclk s(5)Finally, the filter is terminated using a sample and holdbuffer. The circuit schematic for the operational amplifier usedwithin all the switched capacitor circuits is shown in Fig. 4.C. Voltage to Current ConverterThe output stage of this processor is required to provide acurrent output to feed the electrical stimulation subsystem ofthe vestibular prosthesis. The circuit schematic for a linear Vto I converter is shown in Fig. 5. This uses an input resistorto translate the input voltage to the feedback current. Addi-tionally, an output scaled current mirror is used to minimisepower consumption. The transconductance is given by:Iout =W/L(Q10)W/L(Q9)× (Vin − Vbias)R1= κ(Vin − Vbias) (6)Where: κ=20µS (for R1=500KΩ and (W/L)(Q10/Q9)=10)100/510/380/340/510/5 10/5IBIASIN- IN+Q1 Q2 Q3Q4 Q5Q6 Q7Q820KΩ 1pFVOUTFig. 4. Circuit schematic of the operational amplifier used for the switchedcapacitor buffers. Requires an input bias current of 500nA. The compensatedamplifier has an open loop DC gain of 102.6dB, a unity gain bandwidth of735kHz and phase margin 69.2o.100/510/380/340/510/5 10/5IBIASVBIASQ1 Q2 Q3Q4 Q5Q6 Q7Q8RC CC10/5.5Q9 Q10VINIOUTR1100/5.5500KΩ20KΩ 1pFFig. 5. Circuit schematic of the op-amp based transconductance amplifier(i.e. voltage to current converter). Requires an input bias current of 500nA.83Periodic AC Response10−3 10−2 10−1 100 101 102 103 104 105Frequency (Hz)10.00−10.0−20.0Gain (dB)75.050.025.00−25.0−50.0−75.0Phase (deg) harmonic="0";(VOUT/VIN h=0)  pac dB20(V/V)  harmonic="0";(VOUT/VIN h=0) pac deg(V/V) Fig. 6. Simulated frequency response of the semicircular canal processor.Shown is the gain and phase (bode) plots illustrating the implemented transferfunction characteristics (3 poles, 2 zero’s). The peaks visible at 10kHz aredue to the switched-capacitor system clock.IV. SIMULATION RESULTSThe circuit was simulated using the Cadence Spectre(5.1.41) simulator with foundry supplied BSIM3v3 models.Periodic Steady State (PSS) and Periodic AC (PAC) analysiswere used to simulate the response of the switched capacitorfilter. The results showing the transfer function, are presentedin Fig. 6. The static current consumption is 14.8µA drawnfrom a 3.3V supply.V. CONCLUSIONIn this paper we have presented the design of an integratedcircuit for achieving similar transfer characteristics to thesemicircular canal within the vestibular organ in the inner ear.This system includes the sensor interface for connection to acapacitive MEMS-based inertia sensor, a switched capacitorfilter cascade for achieving a biomimetic transfer functionand a voltage to current converter to feed the stimulationdriver. We have focused on achieving low power operationwith good reliability and robustness, whilst additionally beingtunable. Moreover, a fully-integrated implementation alleviatesthe need for off-chip components. A test platform is currentlybeing developed to measure the integrated circuit’s perfor-mance. The microphotograph and floorplan of the fabricatedcircuit is shown in Fig. 7.ACKNOWLEDGMENTThis work was supported by the Cyprus Research PromotionFoundation (RPF), grant no.: Π∆E-0505/07.Vestibular ProcessorAnalogue Padring SectionDigitalPadringSectionHigh-pass Stage C to VInterfaceGain Stage Low-pass Stage GainStage V to I ConverterPower & Clock RailsFig. 7. Microphotograph and floorplan of the fabricated circuit. The circuitoccupies an area of 950µm×1050µm (including ESD I/O cells and bondpads).REFERENCES[1] T. G. Constandinou, J. Georgiou, and C. Toumazou, “A MicropowerFront-end Interface for Differential-Capacitive Sensor Systems,” Proc.IEEE ISCAS, pp. 2474–2477, 2008.[2] T. G. Constandinou, J. Georgiou, and C. Toumazou, “A Partial-Current-Steering Biphasic Stimulation Driver for Neural Prostheses,” Proc. IEEEISCAS, pp. 2506–2509, 2008.[3] J. Georgiou, and C. Toumazou, “A 126µW Cochlear Chip for a TotallyImplantable System,” IEEE JSSC, vol. 40, no. 2, pp. 430–443, 2005.[4] G. Ciaravella, C. Laschi, and P. Dario, “Biomechanical Modeling ofSemicircular Canals for Fabricating a Biomimetic Vestibular System,”Proc. IEEE EMBC, pp. 1758–1761, 2006.[5] A. M. Shkel, and F-G. Zeng, “An Electronic Prosthesis Mimickingthe Dynamic Vestibular Function,” Audiology and Neurotology, vol. 11,pp. 113–122, 2006.[6] W. Gong and D. Merfeld, “A prototype neural semicircular canalprosthesis using patterned electrical stimulation,” Annals of BiomedicalEngineering, vol. 28, pp. 572–581, 2000.[7] C. C. Della Santina, A. A. Migliaccio, and A. H. Patel, “A MultichannelSemicircular Canal Neural Prosthesis Using Electrical Stimulation toRestore 3-D Vestibular Sensation,” IEEE Trans. Biomed. Eng., vol. 54,no. 6, pp. 1016–1030, 2007.[8] C. Toumazou, F. J. Lidgey and D. G. Haigh, Analog IC Design: TheCurrent-Mode Approach. London: Peter Perigrinus, 1990.[9] P. E. Allen, E. S. Sinencio, Switched Capacitor Circuits. KluwerAcademic Publishers, 1984.[10] C. Toumazou, J. B. C. Hughes and N. C. Battersby, eds., Switched-currents: An Analogue Technique for Digital Technology. IEE Publish-ing, 1993.[11] J. J. Groen, “The Semicircular Canal System of the Organs of Equilib-rium - I,” Physics in Medicine and Biology, vol. 1, pp. 103–117, 1956.[12] C. Fernandez and J. M. Goldberg, “Physiology of peripheral neuronsinnervating semicircular canals of the squirrel monkey. II. Response tosinusoidal stimulation and dynamics of peripheral vestibular system,”The Journal of Neurophysiology, vol. 34, no. 4, pp. 661–675, 1971.[13] K. Nagaraj, “A parasitic-insensitive area-efficient approach to realizingvery large time constants in switched-capacitor circuits,” IEEE TCAS-I,vol. 36, no. 9, pp. 1210–1216, 1989.84

A Fully-Integrated Semicircular Canal Processor for an Implantable Vestibular Prosthesis

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