IEEE Solid-State Circuits Magazine - Fall 2014 - 60

Year Author

Link
Speed

Up/Down

Tx/Rx
Power

Skin
Thickness

Misalignment

BER

Guillory EMBC 04

80 Mb/s

90 mW

3 mm

2 mm

<10-14

Abita JHATD 04

115 Kb/s

6.9 mm

Okamoto JSAO 05

9.6 Kb/s

162 mW

20 mm

15 mm

Ackermann TBME 08

40 Mb/s

3 mm

2 mm

<10-5

Parmentier BioCAS 08

16 Mb/s

16 mW

4 mm

2 mm

<10-9

Song TNSRE 09

13.56 Mb/s

2 mm

Liu EMBC 12

50 Mb/s

4.1 mW

4 mm

2 mm

<10-5

Lange ISSCC 11

2 Mb/s

270 µW

Eye (25 mm)

<10-7

Figure 6: State of the art in optical data telemetry.

Especially there, the concept has
been variously employed with tremendous data rates up to almost
100 Mb/s through skin thicknesses
of several millimeters. Figure 5
shows one of our own experiments
for an optical uplink data transmitter. A very simple, current mirrorbased integrated driver has been
combined with a miniature 850-nm
VCSEL bonded onto the ASIC, and
the setup was tested over a 2-6 mm
thick porcine skin. Therewith, we
could achieve up to 50 Mb/s with
only 1 mA from a 2.3-V supply.

Figure 6 shows a state-of-the-art
summary of optical data links. Data
rates of up to 80 Mb/s have been
reported through several millimeters
of skin thickness; while the power consumption was tremendously reduced
in recent versions, the technique still
shows room for improvement.

System Example
A more complex system example
will conclude this article. An epretinal prosthesis is shortly reviewed,
which uses a selection of the concepts discussed here. The principal

Internal RF Link, Rectifier, and Buffer

Stimulator ASIC

External RF Link
and PA

Pt Electrodes
on Retina's Fovea Centralis
Glasses with Camera,
DSP, and IR Diode

Figure 7: An epiretinal stimulator system overview [7].

fa l l 2 0 14

IEEE SOLID-STATE CIRCUITS MAGAZINE

idea of the system in Figure 7 is to
artificially generate light perception
through functional electrical stimulation of the retina. When we look
to the electrical requirements of the
stimulator, reported specifications
of stimulation currents range from
some hundred µA up to mA level
into electrodes with impedances in
the range of up to 10 kΩ. This yields
to worst case 10 20-V stimulation
compliance voltage in such an IMD.
A schematic with a system setup and
operation is illustrated in Figure 8. An
inductive power link at 13.56 MHz is
employed, which is also reused for the
data uplink at 20 Kb/s via LSK. The
rather high-speed downlink at 1 Mb/s
is implemented optically, which physically avoids interference with the
power link.
The power link then feeds two
rectifiers. A high-voltage symmetrical rectifier is used for generation of
the high stimulation supply, which is
then regulated via an adaptive shunt
regulator. Second, we use a second
coil with a lower number of turns to
generate the low-voltage 3.3-V supply
from an adaptable LV rectifier. A digital controller receives data, and the
individual stimulation pixels are programmed under normal operation.
On the telemetry and power management side, three challenges are
highlighted. First, the optical receiver

Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Fall 2014