IEEE Solid-States Circuits Magazine - Fall 2020 - 43

Optical
Filter
Piezo

O2-Sensing Film

µLED

LED
Holder

Thickness = 1.2 mm
Flex
Board

IC

4.5 mm
3 mm
Volume ~4.5 mm3

Encapsulation
(a)

(b)

FIGURE 11:  (a) An exploded view of (b) the implantable O2 sensor designed by University of California, Berkeley. Adapted from [21].

Sensor IC

Sensor
Piezo

Vrect

φmod Active Rectifier

Cstore

Vdc-dc

Modulation
Switch

Voltage Doubler
Power on Reset

RO

Clock
Clock
Division
φref
φref

Bias and Reference
LDOs

LED
Driver

OOK Demod.
VOOK
POR
Analog
PD

φmod
FSM

CLK
10-bit

AFE Vmid
φref

TDC

Data

ILED

ILED

(a)

O2 Concentration (mmHg)

1.96 mm

1.96 mm

path between the LED driver and
the AFE. The power management
blocks consist of 5-V CMOS devices
to withstand a wide range of battery
voltages for regular silicon LEDs and
organic LEDs. The AFE consists of
two stages: a TIA and a variable-gain
amplifier (VGA). The purpose of the
TIA is to sense the current generated by the photodiode and convert
it into voltage. The VGA provides the
additional tunable gain required to
measure low-intensity input signals.
The film's fluorescence can be
measured in terms of either intensity or lifetime, as presented in [39].
The performance of the two techniques was analyzed and compared
in [38]. The lifetime (decay time)
and intensity (amplitude) were measured for different concentrations
of O2, ranging from 0.5 to 500 mm
of mercury. Figure 14(a) and (b) conveys the result for both the low and
high reflectance, respectively. This
simulates the real-world example of
the instrument being used on individuals with different skin tones.
While both the intensity and the
lifetime are inversely proportional
to the concentration of O2, which
can be observed from Figure 14(c),
the intensity of the reemitted light
varies strongly with the differences
in the optical path. The differences
in the lifetime measurement are

NEOFOX O2 Probe
Wireless Sensor

80
60
40
20
0
0

10

20
30
Time (min)

(b)

40

(c)

FIGURE 12:  (a) A system diagram of an implantable O2 sensor. (b) A micrograph of an SoC
in a 65-nm, low-power-CMOS process. (c) The system response to alternating streams of O2
and nitrogen. Adapted from [21]. AFE: analog FE; FSM: finite state machine; RO: ring oscillator;
OOK Demod.: on/off keying demodulation; POR: power on reset; Ref.: reference; TDC: time-todigital converter; D-LDO: digital low-dropout regulator; A-LDO: analog low-dropout regulator.

	 IEEE SOLID-STATE CIRCUITS MAGAZINE	

FA L L 2 0 2 0	

43



IEEE Solid-States Circuits Magazine - Fall 2020

Table of Contents for the Digital Edition of IEEE Solid-States Circuits Magazine - Fall 2020

Contents
IEEE Solid-States Circuits Magazine - Fall 2020 - Cover1
IEEE Solid-States Circuits Magazine - Fall 2020 - Cover2
IEEE Solid-States Circuits Magazine - Fall 2020 - Contents
IEEE Solid-States Circuits Magazine - Fall 2020 - 2
IEEE Solid-States Circuits Magazine - Fall 2020 - 3
IEEE Solid-States Circuits Magazine - Fall 2020 - 4
IEEE Solid-States Circuits Magazine - Fall 2020 - 5
IEEE Solid-States Circuits Magazine - Fall 2020 - 6
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IEEE Solid-States Circuits Magazine - Fall 2020 - Cover3
IEEE Solid-States Circuits Magazine - Fall 2020 - Cover4
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