IEEE Solid-States Circuits Magazine - Fall 2020 - 39

analysis. Transcutaneous blood gas
monitoring using electrochemical
probes and bedside units has been
quite successful in the past. However,
the presence of a heating element
has negatively affected the transition of the technology from bedside
monitoring to a long-term wearable.
Designing a heating element on a
wearable will drastically increase the
device size and power requirements,
limiting the -wearable's use for continuous, long-term monitoring. In
this section, we discuss emerging
next-generation blood gas monitoring systems, including transcutaneous and PPG, particularly monitoring
O2 content.

measured with a PD, as depicted in
Figure 5. When the dye is exposed to
a short wavelength (i.e., blue light), it
reemits light at a longer wavelength
(red/orange). The intensity and lifetime of the luminescence are dependent on the concentration of O2. In
low-O2 environments, a large number of photons are emitted by the
film. In the presence of O2, the light
is quenched as energy is transferred
to the O2 molecule instead of being
emitted as a photon. A PD circuit
measures the emitted photons' lifetime (or intensity) to determine the
O2 concentration in the surrounding
environment of the dye.

Emerging Next-Generation
Blood Gas Monitors

Featured PPG-Based Monitors

Despite the massive efforts demonstrated in creating a new portfolio
of next-generation smart and connected biomedical devices, limited
studies have been presented for
wearable/implantable -technology
in the critical field of blood gas

PPG-Based Monitor in a System-onChip With Bluetooth Low Energy
Work presented in [45] proposed an
all-in-one, battery-powered systemon-chip (SoC) health-monitoring patch
for continuous evaluation in a home

Readout FE

LED
Driver

5 mA × 32

VBIA SN

ADC

VINP

Signal
Current
N3
l1

N4
l2

-A
+ 2
Local VBIA SN
Feedback
Main
Feedback

Current-Sensing Stage

VOPCM

VINN

VOP
+ A
- 3

PPG
CIC/FIR

dc Filter/
Register

IDAC

-
A1 +
VBIA SP Local
Feedback
Main
Feedback

+
TIA /
INT

VBIA SP

Digital

+
Current
Sensing
-
+

setting. The SoC is designed in a
55-nm technology node that provides
an optimum solution for the cointegration of advanced low-power analog/radio frequency (RF) and digital
blocks. A system diagram of the PPG
readout is presented in Figure 6. The
PPG readout consists of a currentsensing stage and a fully differential
transimpedance stage. A 16-b analogto-digital converter (ADC) with a digital-assisted dc-current-cancellation
loop follows the amplifier to maintain
a high dynamic range. The dc-cancellation loop helps to reduce the dc
component (the ambient component)
in the signal, facilitating a higher ac
gain and more dynamic range.
The SoC was integrated into a
disposable medical-grade encapsulation, as presented in [26] and shown
in Figure 7. The wearable health patch
incorporates a pulse oximeter, a bioimpedance sensor, a thermometer,
and an ECG. The system integrates the
SoC, external electrodes, LEDs, PDs,
Bluetooth Low Energy (BLE) antenna,

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Signal
Current

N2
VCG

VONCM

VOPCM

VON
A4 +
-

VREFO
VONCM

TIA Stage With CMFB

FIGURE 6: A system diagram of the PPG readout front end (FE). Adapted from [26]. TIA: transimpedance amplifier; ADC: analog-to-digital
converter; CIC/FIR: cascaded integrator-comb/finite impulse response; IDAC: inter-digital analog converter; CMFB: common-mode feedback;
INT: integer.

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IEEE Solid-States Circuits Magazine - Fall 2020

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