biotelemetry: a review of the art and some interesting circuits for low-power, low- noise frequency...

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

September, 25, 2003 The Krasnow Institute Seminar

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


What is it and why would you 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

Why 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.


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)

Temperature EMG, Motor activity EEG ECG, Heart rate EOG Pressure (e.g. arterial, venous, left ventricular, intra-

ocular, bladder, & kidney) - Transoma Medical

Measurements

Mini Matter: “Pediatric to geriatric, mice to men, miniaturized [biotelemetry] products from Mini Mitter are appropriate for all research subjects.”


Wearable Devices Mini Mitter’s Actiwatch TM - Medical diagnostics

Mini Mitter’s Actiwear TM - Periodic Limb Movement for diagnosing sleep disorders

Mini Matter’s Actical TM - Accelerometer for diagnosing obesity, nutrition, exercise, and rehabilitation

Externally worn devices do not need to be extremely low- power.


Commercial Devices for Physiological Monitoring

Mini Mitter’s Vital View TM

Body Core Temperature Heart Rate Gross Motor Activity Running Wheel Turns Drinking/Licking Frequency Feeding Behavior Ambient Temperature Ambient Light

Implantable transmitters

(temperature and gross motor movement)

Implantable “e-mitters”

(heart rate and movement monitoring)

Receiver

Companies:

• Biotelemetrics, Inc.

• Kent Scientific

• Mini Mitter

• Spacelabs

• Transoma Medical (formerly Data Sciences Int’l.)


Prior Work Monitoring and Recording

Neurostimulation

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

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


Typical Specifications

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!)

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: -20-50C, Implants: 30-45C ? Battery Life: 100s of hrs to several years Packaging Materials: PECVD silicon dioxide, silicon nitride, DLC,

parylene


Telemetric Transceiver - Block Diagram

Sensor/Preamp n

Data Conv.

RectifierRegulator

MUX

Sensor/Preamp 1

Power

Inductive Power/Data

Link

(E.g. sigma-delta modulators)

LO

(Xtal, SAW, or PLL)

Tissue interface

• Piezoresistive accelerometers

• SAW resonators

• Thermistors

• Pressure sensors

• Ion concentration sensors

• Micro-electrode arrays

M

LextLint

Mod/Dem: PCM, PPM, PSK, etc.


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)


Data Conversion: Modulators

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

+ 1/sSensed analog

voltage in

Bit stream

out

8

5

A.K.A. The Line-Draw Algorithm: How to get from point A to point B in the straightest line on a Manhattan grid:

clock

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

Example: analog input 5/8 of full-scale:


Frequency Synthesis: Multiplying PLL

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).

H(s)+ Kv/s

(φo φi)

(ωo Mωi)

Vvco=φo

Freq. Divider (M)

Vi cos(ωi t+φi) Vo cos(ωot+φo)(φe 0)

ωf = ωo /M


Vod cos(ωrt)

Voltage-Controlled Oscillator

finite Q

ωr=1/ LC

RlossCL -Rloss

M1

M2

M3

M4

EQUIVALENT TANK CKT:C

L

Rs Rs

1/f 3

1/f 2

Δf

dBc/Hz

Phase Noise: LC or ring-oscillator? LC-based VCOs are much less noisy than ring-oscillators power reduction for given noise performance

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).

RlossC2L2-RlossRloss C1 L1

-RlossV1

+

-V2

+

-

Mutual inductance, M

1/f up-conversion

capacitor

PHASE NOISE PSD:


Half-ckt, small-signal analysis (negative resistance cancels loss in tank) :

Vod cos(ωrt)Vod /2

T-model

-Vod /2

i1=gm1(Vod /2-Vs)

Negative Resistance

ro1

1/gm1

Rs

io = (approx i1)

ro1i1=Gm1 (-Vod /2)

Gm1 = gm1/(1+ gm1 Rs)

SOURCE-DEGENERATED EQUIVALENT CKT:

=>Req=-1/Gm1= -Rloss

at resonance

M1

M2

M3

M4

=>

C

L

Rs Rs

Vs


Magnetically Coupled LC VCOs

V1 = - I1(sL+1/sC) = MI2s

Letting L= L1 = L2 and C= C1 = C2 => ωr= ωr1= ωr2 M = k sqrt(L1L2) = kL

V2 = - I2(sL+1/sC) = MI1s

and solving the following simultaneous equation:

yields two steady-state solutions, or modes:

V1

V2

=V2

V1

= -1 (odd mode, V1 and V2 oscillate out of phase)

V1

V2

=V2

V1

= 1 (even mode, V1 and V2 oscillate in phase)

ωr2=1/sqrt(L2C2)ωr1=1/sqrt(L1C1)

RlossC2L2-RlossRloss C1 L1

-RlossV1

+

-V2

+

-

Mutual inductance, M

Advantage: Common-mode disturbances at ωo

are attenuated! More coupling => more attenuation.

ωo= ωr(1+.5M/L)

ωe= ωr(1 - .5M/L)

Stand-alone res. freqs ->

ωr ωoωe

odd mode

even mode


Injection-Locked Frequency Divider

Ideally, ωr=2ωi, but it will not in practice, so we need to pull the oscillator before phase-locking.

Non-linearity is used to pull and injection-lock an otherwise free-running oscillator at its natural frequency, ωr:

H(s) =sHoωr/Q

s2+s ωr/Q + ωr2

;Δωr = ωo-ωr (how far the oscillator is currently off the BP resonance)

Vo cos(ωot) f(x)+

f(x)=a1x+ a2x2

Vi cos(ωit + φ)H(ω)=BPF

Injection source

oscillator

ωr

Vf

Ho

1+j2QΔωr/ωr

Also define: Δωe= ωo - ωi /2

(how far the output freq. is currently from half the input freq.)

H(jω) =


Vo = H(jωo)Vf =Ho Vf

1+j2QΔωr/ωr

Output = Input x H(jωo):

;Vf (t)=a1Vo cos(ωot) + a2ViVo cos((ωo - ωi)t - φ)

1+j2QΔωr /ωr = Ho [a1 + a2Vie jφ]

Equating imaginary parts:

Using complex exponentials and neglecting Δωe:

Δωr/ωr = .5Ho a2Visin(φ)/Q

The maximum locking range corresponds to when sin(x)=1 => Δωr /ωr < Ho a2Vi /2Q

If ωr is perfectly matched to ωi /2 then s.s. phase error is zero.

Vo cos(ωot)

f(x)+

f(x)=a1x+ a2x2

Vi cos(ωit + φ)H(ω)=BPF

Injection source

oscillator

ωr

Vf

Lock Range


Vi cos(ωit)

Vod cos(ωot+ φo) L L

CCVs


M1

M2

M3

Vod +-

ωr=1/sqrt(LC)

RlossCL -Rloss

Small-signal half-ckt:

Vod /2

ro1

1/gm1

T-model

-Vod /2

i1=gm1(Vod /2-Vs)

i3=gm3Viro3

ioUsing driving-point impedance, inspection, or other analysis technique:

io= - Gm1 Vod /2 + gm3ro2 Gm1 Vi

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...

EQUIVALENT TANK CKT:


Consider a pseudo-large-signal half-ckt analysis (i.e. still use small-signal approximation for tail current source, M3):

Idsat=[.5μCoxW/L](VGS -Vt)2 ; VGS= VG- VS = VGSDC

+ VGSAC

Io

M1

M3VI =VIDC

+ VIAC

VG= VGDC+ VGAC

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) ]]

Large-signal 2nd-order non-linearity comes from square-law:

Thus, the linear mixture of oscillator frequency and injection source is mixed via 2nd-order nonlinearity, producing intermodulation terms near ωr, which is intentionally placed near ωi /2. This energy serves to injection lock the oscillator.



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!

biotelemetry: a review of the art and some interesting circuits for low-power, low- noise frequency...

Documents

mortality n temperature

n summary slide

motor activity n eeg

heart rate n eog n pressure

n sympathetic nerve

physiological data

postdoctoral candidate

low power