Challenges in Realizing a Chronic High-Resolution Retinal Prosthesis - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 146

placed orthogonal to the power coils L 1_pwr and L 2_pwr , which ideally reduce the
coupling coefficients of K 11 and K 22 to zero. However, winding L 2_data around
L 2_pwr (to make the zero coupling of K 22 ) creates a bump on coil L 2_pwr , which
is undesirable from the surgical perspective.
Compared to the single-band approach, the dual-band approach requires one
more coil to be implanted, which increases the size of the implant. On the other
hand, it can save power since it allows power link to be optimized without
sacrificing for data transmission. Design of a dual-band telemetry system should
try to minimize the size increase due to the extra coil.
Reverse Data Telemetry Design
Reverse telemetry may use another frequency as data carrier, similar to forward
data telemetry. This approach is called 'active reverse telemetry'. The data carrier
may use the existing coil pair or another coil pair as physical link. In general, this
approach requires an active driver to drive the coil/antenna. Another approach is
called “passive reverse telemetry,” as it does not require a dedicated data driver.
Figure 7.11 shows the principle of using passive telemetry to transmit data from
the implant to the external unit. It should be noted that in a dual-band telemetry
system, the reverse telemetry can use the power telemetry as its data carrier, since
it does not require high data rate and the low frequency power carrier is a good
candidate. As shown in Figure 7.11, the coil is not driven by any power amplifier.
Instead, a change in the loading of the coil is introduced by turning on and off the
switch S. This loading change at the secondary coil creates a detectable change
in the coil current (hence called load shift keying [22]). A receiver in the external
unit connected to the primary coil detects this change and recovers the data. The
load shift keying shown in Figure 7.11 may generate a high voltage stress on
the switch and the diode, when the switch is open, since interrupting current
flow through an inductor produces a large voltage to counteract the change. This
high voltage in turn makes it difficult to implement the switch in the IC, if it is
above the operating voltage of the chosen process technology, and may require
discrete components, which increases the size of the implant. Thus the reverse
telemetry should be designed to maintain the advantage of low power dissi-
pation while minimizing the discrete components and consuming as less space as
possible.
Figure 7.11. Passive reverse telemetry.