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Biotelemetry: A Review of the Art and Some Interesting Circuits for Low-Power, Low-Noise Frequency Synthesis

Biotelemetry: A Review of the Art and Some Interesting Circuits for Low-Power, Low-Noise Frequency Synthesis. Ron Spencer, Ph.D. Postdoctoral Candidate Seminar September 25, 2003. Preview. Biotelemetry: What is it and why use it? Prior Work & Typical Specs

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Biotelemetry: A Review of the Art and Some Interesting Circuits for Low-Power, Low-Noise Frequency Synthesis

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  1. Biotelemetry: A Review of the Art and Some Interesting Circuits for Low-Power, Low-Noise Frequency Synthesis Ron Spencer, Ph.D. Postdoctoral Candidate Seminar September 25, 2003

  2. Preview • Biotelemetry: What is it and why use it? • Prior Work & Typical Specs • Basic architecture of Telemetric Transceiver • Power Reduction Strategies • Data Conversion(using  converters) • Frequency Synthesis:- PLLs- VCOs(using magnetic coupling)- Frequency division(using injection-locking) • Summary The Krasnow Institute Seminar

  3. What is it and why would you use it? Biotelemetry is the wireless transmission of automatically measured physiological data from the point of sensing to a remote location. However, in practice the term also refers bidirectional wireless data transfer and remote powering. Why Use It? • Necessity - Implantable prosthetics - retinal, cochlear, pacemaker - Animal tracking - freedom of movement • Quality of measurement -  tethering forces on electrodes • Convenience - Multi-electrode stimulation/recording - Vital sign monitoring in critical situations The Krasnow Institute Seminar

  4. Applications • Neuroprosthetics (neurostimulation) • Multi-electrode recording • Vital sign monitoring in critical and ambulatory care • Vital sign monitoring of pilots and astronauts • Remote wildlife monitoring and tracking (avian, fish, mammals, reptiles - activity, depth, altitude, temperature, mortality) Measurements • Temperature • EMG, Motor activity • EEG • ECG, Heart rate • EOG • Pressure (e.g. arterial, venous, left ventricular, intra-ocular, bladder, & kidney) - Transoma Medical Mini Matter: “Pediatric to geriatric, mice to men, miniaturized [biotelemetry] products from Mini Mitter are appropriate for all research subjects.” The Krasnow Institute Seminar

  5. Wearable Devices Mini Mitter’s Actiwatch TM - Medical diagnostics Mini Mitter’s Actiwear TM - Periodic Limb Movement for diagnosing sleep disorders Externally worn devices do not need to be extremely low- power. Mini Matter’s Actical TM - Accelerometer for diagnosing obesity, nutrition, exercise, and rehabilitation The Krasnow Institute Seminar

  6. Commercial Devices for Physiological Monitoring Companies: Mini Mitter’s Vital ViewTM • Body Core Temperature • Heart Rate • Gross Motor Activity • Running Wheel Turns • Drinking/Licking Frequency • Feeding Behavior • Ambient Temperature • Ambient Light • Biotelemetrics, Inc. • Kent Scientific • Mini Mitter • Spacelabs • Transoma Medical (formerly Data Sciences Int’l.) Implantable transmitters (temperature and gross motor movement) Receiver Implantable “e-mitters” (heart rate and movement monitoring) The Krasnow Institute Seminar

  7. Prior Work • Sieve electrode recording (Akin et al.) • Sympathetic nerve activity and ECG measurement (Enokawa, et al.) • Auditory experimentation (Lukes, et al.) • Monitoring of freely moving animals and insects • Single neuron discharge in monkeys • Monitoring and Recording • Neurostimulation • Retinal prosthesis - retinitis pigmentosa and macular degeneration - MIT - 2nd Sight (Alfred Mann Foundation) - Gregg Suaning, U. New South Wales (100 ch. Bidirectional RF-CMOS) • Cochlear prosthesis - Advanced Bionics Corporation • Pacemakers The Krasnow Institute Seminar

  8. Typical Specifications • Size: 10s of mm side length (contrast with typical IC sizes of 4-30 sq. mm.) • Weight: 1-40g • Inductor sizes: mm (half-wavelength) (motivation for higher carrier frequencies: λ = c / εr / f ) • RF link operating distance: cm to meters • Power consumption: mW • Power supplies: (set by technology): 3.3V - 1.0V • Temperature Range: Wildlife apps: -20-50C, Implants: 30-45C ? • Battery Life: 100s of hrs to several years • Packaging Materials: PECVD silicon dioxide, silicon nitride, DLC, parylene • Input Impedance: up to 1G ohms, 10pF (e.g. for good voltage xfer from electrodes) • Sensitivity: mV • Channel bandwidth: 100-100kbs, 1Mbs needed • Carrier frequencies: 1-200MHz (contrast with fund. mode crystals up to around 40MHz) • LO Phase Noise: -100dBc/Hz spot noise at 500kHz offset from carrier (contrast with state-of-the-art optical communications: -100dBc/Hz at 100kHz from 2.5GHz  approx. 32dB lower!) The Krasnow Institute Seminar

  9. Telemetric Transceiver - Block Diagram Regulator Rectifier Power Mod/Dem: PCM, PPM, PSK, etc. Sensor/Preamp 1 M Inductive Power/Data Link Data Conv. MUX Lint Lext Sensor/Preamp n (E.g. sigma-delta modulators) LO • Piezoresistive accelerometers • SAW resonators • Thermistors • Pressure sensors • Ion concentration sensors • Micro-electrode arrays (Xtal, SAW, or PLL) Tissue interface The Krasnow Institute Seminar

  10. Power Reduction Strategies • Devise low power standby modes (turn circuits off when not in use) • Smaller technology & lower power supply voltage • Inductive power coupling (>70% efficiency) • Low-power mod/demod; e.g. PCM • Reduce RF data link operating distance • Magnetically-coupled oscillators (instead of shielding) • Injection-locked frequency division(on the order of 6dB power reduction over brute-force methods) The Krasnow Institute Seminar

  11. + 5 8 Data Conversion:  Modulators Bit stream out Sensed analog voltage in 1/s clock • Similar to integrate-and-fire neuron • Very simple to implement • Very high resolution at audio frequencies (up to 20 bits) • Oversampling pushes quantization noise out to high frequencies (noise shaping) • Insensitive to many analog non-idealities Example: analog input 5/8 of full-scale: A.K.A. The Line-Draw Algorithm: How to get from point A to point B in the straightest line on a Manhattan grid:  • 0 - 0 + 5 = 5 (< 8  quiet  move over  0) • 5 - 0 + 5 = 10 (>=8  fire  move up  1) • 10 - 8 + 5 = 7 (< 8  quiet  move over  0) • 7 - 0 + 5 = 12 (>=8  fire  move up  1) • 12 - 8 + 5 = 9 (>=8  fire  move up  1) • 9 - 8 + 5 = 6 (< 8  quiet  move over  0) • 6 - 0 + 5 = 11 (>=8  fire  move up  1) • 11 - 8 + 5 = 8 (>=8  fire  move up  1) 5 pulses out of 8 The Krasnow Institute Seminar

  12. Vvco=φo (φe 0) + (φo φi) Frequency Synthesis: Multiplying PLL Vo cos(ωot+φo) Vi cos(ωi t+φi) H(s) Kv/s • PLLs drive the phase of an oscillator to be some fixed offset from that of the input. • Ideally, the resonant frequency of the VCO is exactly M times ωi. If not, the VCO is adjusted to the correct frequency by H(s). (ωo Mωi) Freq. Divider (M) ωf = ωo /M The Krasnow Institute Seminar

  13. Mutual inductance, M + + V2 V1 C2 -Rloss Rloss Rloss -Rloss C1 L1 L2 dBc/Hz - - 1/f 3 1/f 2 Δf Voltage-Controlled Oscillator Phase Noise: LC or ring-oscillator? LC-based VCOs are much less noisy than ring-oscillators  power reduction for given noise performance Vod cos(ωrt) PHASE NOISE PSD: EQUIVALENT TANK CKT: 1/f up-conversion M4 M2 C finite Q Rloss -Rloss L C capacitor L M3 M1 ωr=1/ LC Rs Rs Immunity to EMI: To shield or not to shield? Shielding via low- metal reduces the Q and increases power for given noise performance. Magnetic coupling can de-tune far-field (even-mode) response curve away from near-field (odd-mode). The Krasnow Institute Seminar

  14. Negative Resistance Half-ckt, small-signal analysis (negative resistance cancels loss in tank) : Vod /2 Vod cos(ωrt) T-model M4 M2 C i1=gm1(Vod /2-Vs) ro1 => -Vod /2 L 1/gm1 M3 M1 Vs Rs Rs Rs SOURCE-DEGENERATED EQUIVALENT CKT: io = (approx i1) ro1 =>Req=-1/Gm1= -Rloss at resonance i1=Gm1 (-Vod /2) Gm1 =gm1/(1+ gm1 Rs) The Krasnow Institute Seminar

  15. Mutual inductance, M + + V2 V1 C2 -Rloss Rloss Rloss -Rloss C1 L1 L2 - - Magnetically Coupled LC VCOs ωr1=1/sqrt(L1C1) Stand-alone res. freqs -> ωr2=1/sqrt(L2C2) Letting L= L1 =L2 and C= C1 =C2 => ωr=ωr1=ωr2  M = k sqrt(L1L2) = kL and solving the following simultaneous equation: even mode odd mode V1 = - I1(sL+1/sC) = MI2s V2 = - I2(sL+1/sC) = MI1s yields two steady-state solutions, or modes: ωr ωe ωo V1 V2 = = -1 (odd mode, V1 and V2 oscillate out of phase) Advantage: Common-mode disturbances at ωo are attenuated! More coupling => more attenuation. V2 V1 ωo= ωr(1+.5M/L) V1 V2 = = 1 (even mode, V1 and V2 oscillate in phase) V2 V1 ωe= ωr(1 - .5M/L) The Krasnow Institute Seminar

  16. H(ω)=BPF f(x)=a1x+ a2x2 Vi cos(ωit +φ) Vf f(x) Injection source ωr oscillator + sHoωr/Q H(s) = s2+s ωr/Q + ωr2 Ho 1+j2QΔωr/ωr Injection-Locked Frequency Divider Vo cos(ωot) Non-linearity is used to pull and injection-lock an otherwise free-running oscillator at its natural frequency,ωr: Ideally,ωr=2ωi, but it will not in practice, so we need to pull the oscillator before phase-locking. H(jω) = Also define: Δωe= ωo - ωi /2 (how far the output freq. is currently from half the input freq.) ;Δωr = ωo-ωr(how far the oscillator is currently off the BP resonance) The Krasnow Institute Seminar

  17. H(ω)=BPF f(x)=a1x+ a2x2 Vi cos(ωit +φ) Vf f(x) Injection source ωr oscillator + Ho Vf Vo = H(jωo)Vf = 1+j2QΔωr/ωr Lock Range Vo cos(ωot) Output = Input x H(jωo): ;Vf (t)=a1Vo cos(ωot) + a2ViVo cos((ωo -ωi)t - φ) Using complex exponentials and neglecting Δωe: 1+j2QΔωr /ωr = Ho [a1 + a2Vie jφ] Equating imaginary parts: Δωr/ωr = .5Ho a2Visin(φ)/Q If ωr is perfectly matched to ωi /2 then s.s. phase error is zero. The maximum locking range corresponds to when sin(x)=1 => Δωr /ωr < Ho a2Vi /2Q The Krasnow Institute Seminar

  18. Rloss -Rloss L C ωr=1/sqrt(LC) Injection-Locked Frequency Divider Vod cos(ωot+ φo) EQUIVALENT TANK CKT: L L Vod - + C C M1 Vs M2 Vi cos(ωit) M3 Small-signal half-ckt: Using driving-point impedance, inspection, or other analysis technique: Vod /2 io T-model i1=gm1(Vod /2-Vs) ro1 -Vod /2 io= - Gm1 Vod /2 + gm3ro2 Gm1 Vi 1/gm1 Thus, the output current is a mixture of output frequency and injection source. To see the pulling and injection-locking, we must consider large-signal effects... ro3 i3=gm3Vi The Krasnow Institute Seminar

  19. Io VG= VGDC+VGAC M1 VI =VIDC+ VIAC M3 Injection-Locked Frequency Divider Large-signal 2nd-order non-linearity comes from square-law: Idsat=[.5μCoxW/L](VGS -Vt)2 ; VGS= VG-VS = VGSDC+ VGSAC Consider a pseudo-large-signal half-ckt analysis (i.e. still use small-signal approximation for tail current source, M3): AC components of VGS: VGAC= -Vod /2, VSAC= VIAC gm3/(go3+ gm1) DC AC (VGS -Vt)2=[[VGSDC-Vt]- [Vod /2+ VIAC gm3/(go3+ gm1)]] Thus, the linear mixture of oscillator frequency and injection source is mixed via 2nd-order nonlinearity, producing intermodulation termsnearωr, which is intentionally placed near ωi /2. This energy serves to injection lock the oscillator. The Krasnow Institute Seminar

  20. Summary • Biotelemetry is useful for - Neural stimulation (data in  device) - Neural recording (device  data out) - Eliminating tethering forces -Vital sign monitoring - Animal tracking • Biotelemetry represents a multidisciplinary research area that enables collaboration and offers a diverse EE design experience: • ckt design • EM • communications • sensor design • transmission-line/waveguide design • antenna design • Today’s CMOS technology may be leveraged to reduce power and size, improve performance, and increase throughput References available upon request. THANK YOU! The Krasnow Institute Seminar

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