Biomedical Engineering Reference
In-Depth Information
the.standard.approach.of.using.two.regulated.power.supplies,.the.cascoded.resonant.tank.
saves.roughly.40%.of.power.without.introducing.additional.off-chip.components.
16.6 WirelessDataTelemetry
Two. pairs. of. coils. for. transferring. power. and. data. are. placed. coaxially. to. increase. cou-
pling. and. reduce. constraints. of. surgical. operation.. However,. this. coniguration. causes.
severe.interference.between.power.and.data.and.may.corrupt.the.data.demodulation..A.
noncoherent.differential.phase.shift.keying.(DPSK).scheme,.where.1.is.coded.as.a.phase.
shift. of. 180°. and. 0. is. coded. as. no. phase. shift,
44
. can. be. used. to. reduce. the. interference.
via.cancelation..With.the.assignment.of.a.data.rate.equal.to.the.power.carrier.frequency,.
two.consecutive.symbols.are.under.the.same.power.interference,.which.is.eliminated.by.
differentiation.
The. implemented. DPSK. receiver. uses. the. subsampling. scheme,
45
. where. the. sampling.
frequency. is. lower. than. the. data. carrier. frequency.. The. adoption. of. this. subsampling.
scheme. is. based. on. two. reasons.. First,. a. low-frequency. operation. reduces. demodulator.
power.. Second,. it. samples. the. band. pass. signal. without. downmixing,. which. involves. a.
phase-locked.loop.circuitry..Equation.16.5.gives.the.relationship.among.the.sampling.fre-
quency.(
f
s
),.data.carrier.frequency.(
f
c
),.and.data.rate.(
f
d
),.ensuring.that.nonzero.crossing.
points.of.the.data.carrier.are.sampled:
f
=
4
f
/(
2
i
+
1
)
=
4
j f
,
i
=
1 2 3
,
,
,
;
j
=
1 2 3
,
,
,
…
…
.
(16.5)
.
s
c
d
Using.
S
i
,
j
.to.denote.the.clock.sequence,.where.
i
.is.the.symbol.index.and.
j
.the.sample.index,.
at.the.end.of.each.symbol,.the.comparator.output.is
∑
V
out
=
|
S
−
+
S
|
.
(16.6)
i
,
i j
,
i
1
,
j
.
j
When.there.is.no.phase.shift,.
S
i
,
j
.-.
S
i
+1,
j
.≈.0.and.
V
i
,out
.is.a.small.value..When.there.is.a.180°.
phase.shift.and.the.sampled.points.are.not.the.zero.crossing.ones,.|
S
i
,
j
.-.
S
i
+1,
j
|.>>.0.and.
V
i
,out
.
becomes.a. large. positive. value.. By. comparing.
V
i
,out
. to.a. predeined. threshold,.the. phase.
shifts.are.identiied.
16.7 StimulatorandDigitalController
An. example. stimulator. pixel. architecture. is. shown. in. Figure 16.8.. It. consists. of. a. local.
digital. controller,. level. shifter,. digital-to-analog. converter. (DAC),. variable-gain. current.
mirror.(VGCM),.protection.circuitry,.driver.stage,.electrostatic.discharge.(ESD).protection.
circuitry,. and. area. pad.. Functionally,. the. LDC. generates. stimulation. waveform. patterns.
based.on.the.decoded.bit.stream.by.the.demodulator.and.GDC..The.level.shifter,.DAC,.and.
VGCM. interpret. the. waveform. patterns. and. control. the. high-voltage. (HV). output. stage.
that.injects.stimulus.through.the.electrode..Any.residual.charge.on.the.electrode.interface.
