ANALOGUE DESIGN WEARABLE MONITORS
various measurement results.
Each time slot starts with a
precondition pulse, followed by
a stimulus pulse and, finally, the
photodiode current or signal from
another sensor signal that is sampled
by the ADC.
After power up, followed by a
reset operation, the chip enters
sleep mode. After waking up the
chip, it’s possible to sequentially
sample two ECG signals, followed by
an optical measurement to perform
a SpO2 reading and an impedance
measurement to measure skin
conductance (EDA/stress).
ECG measurement made easy
An ECG is the measurement of an
electric signal generated by the human
heart, due to depolarization and
repolarization of the heart muscle per
every heartbeat.
The signal is typically 0.5 mV to 4
mV in amplitude and measured over
a frequency range from 0.05 Hz up to
40 Hz. It can be performed to solely
measure heart rate, but practitioners
are more interested in the waveform
itself, which can be used as a
measure for cardiac performance or a
pre-warning to a potential cardiac.
The cardiac activity is monitored
by attaching electrodes to the skin
and for good contact wet electrodes
are typically used. The most popular
are silver/silver-chloride (Ag/AgCl)
electrodes.
In out-of-hospital applications,
these electrodes are very
uncomfortable and can easily dry
out or start to irritate the skin.
Furthermore, though dry electrodes are
often used, the contact between skin
and electrode is degraded leading to
less accurate readings.
The ADPD4000 uses a voltage
input, and the ECG circuit measures
electrical charge accumulated on a
sensing capacitor. With an optimised
time-constant calculated from the
passive RC network and sampling
rate, the charging process eliminates
the variation on the skin-to-electrode
contact impedance. This ECG circuit
has inherent immunity to variations
in the skin-to-electrode contact
impedance.
In Figure 2 two ECG waveforms
are shown. The blue one has been
measured with a good quality
electrode, having a series impedance
of 51 kΩ and 47 nF capacitance. The
red waveform has been measured
with a poor quality electrode and has
a 510 kΩ contact impedance with
4.7 nF capacitance. It is possible to
see that the ADPD4000 is measuring
both waveforms, almost identically,
independent of the electrode quality.
This is a huge advantage of this
front end over other solutions currently
on the market.
An additional advantage is that this
circuit is extremely power efficient,
it doesn’t have to be active while
capturing the ECG signal on the charge
capacitor. Another advantage is that it
consumes 150 μW to 200 μW.
PPG and bioimpedance
For optical and bioimpedance
measurements, LED drivers are
required to emit light and to excite a
current into the body, respectively. In
many optical systems, more than just
one wavelength is used, which makes
the chip’s versatility very desirable.
The ADPD4000 has eight output
drivers, where four channels can
be used simultaneously with a
programmable output current of
maximum 200 mA per channel or 400
mA for the total driver section.
Depending on the configuration, it
is possible to operate multiple time
slots, each with its own wavelengths
to measure optical heart rate, SpO2,
hydration, or dehydration etc.
Each receive signal chain has
a programmable transimpedance
amplifier followed by a dual-stage
rejection block to cancel out ambient
light interferers. The signal-to-noise
ratio (SNR) of the transmit/receive
signal chain is up to 100 dB, which
makes the device useful for noisesensitive
optical measurements such
as oxygen saturation measurement or
blood pressure estimation.
The power consumption of the
optical system very much depends
on the system configuration such as
sampling and decimation rate, and the
LED current used.
Many wearable systems measure
skin conductance for applications
like EDA, stress, or mental-state
monitoring.
An excitation current is needed
in order to measure a voltage drop.
The ADPD4000 supports this and
the chip can be configured in a
2- or 4-wire measuring mode. An
enhanced waveform generator and
DFT engine are not included, so in
case impedance spectroscopy is
required, the AD5940 should be used
as a companion chip to complement
the ADPD4000. The impedance
function also can be used to measure
electrode quality or for lead-off
detection.
Almost ideal?
The ADPD4000/ADPD4001 looks
to address the challenges designers
face when working on wearable
devices, body patches, or drug delivery
systems, where performance, size,
and power dissipation are critical
specifications.
While both the ADPD4000 and
4001 have reached volume production
a next generation, ADPD4100/4101,
is now expected to become available
in the first half of 2020.
Author details
Jan-Hein
Broeders,
Healthcare
Business
Development
Manager, Analog
Devices
Figure 2: Two
ECG waveforms
measured with
different electrodes
www.newelectronics.co.uk 28 April 2020 17
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