Delft University of Technology
A 0.6V 3.8μW ECG/bio-impedance monitoring IC for disposable health patch in 40nm
CMOS
Xu, Jiawei; Lin, Qiuyang; DIng, Ming; Li, Yao; Van Hoof, Chris; Serdijn, Wouter; Van Helleputte, Nick
DOI
10.1109/CICC.2018.8357025
Publication date
2018
Document Version
Accepted author manuscript
Published in
2018 IEEE Custom Integrated Circuits Conference, CICC 2018
Citation (APA)
Xu, J., Lin, Q., DIng, M., Li, Y., Van Hoof, C., Serdijn, W., & Van Helleputte, N. (2018). A 0.6V 3.8μW
ECG/bio-impedance monitoring IC for disposable health patch in 40nm CMOS. In A. Piovaccari, & H. Wang
(Eds.),
2018 IEEE Custom Integrated Circuits Conference, CICC 2018
(pp. 1-4). Institute of Electrical and
Electronics Engineers (IEEE). https://doi.org/10.1109/CICC.2018.8357025
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A 0.6V 3.8μW ECG/Bio-Impedance Monitoring IC
for Disposable Health Patch in 40nm CMOS
Jiawei Xu
1
, Qiuyang Lin
2,3
, Ming Ding
1
, Yao Li
2
, Wouter Serdijn
2
, Chris Van Hoof
3
, Nick Van Helleputte
3
1
imec - Holst Centre, Eindhoven, The Netherlands,
2
Delft University of Technology, Delft, The Netherlands,
3
imec, Leuven, Belgium
Abstract—Simultaneous measurement of Electrocardiogram
(ECG) and bio-impedance (BioZ) via disposable health patches is
desired for patients suffering from chronic cardiovascular and
respiratory diseases. However, a sensing IC must consume ultra
low power under a sub-volt supply to comply with miniaturized
and disposable batteries. This work presents a 0.6V analog front-
end (AFE) IC consisting an instrumentation amplifier (IA), a
current generator (CG) and a SAR ADC. The AFE can measure
ECG and BioZ simultaneously with the single IA by employing a
orthogonal chopping scheme. To ensure the IA can tolerate up to
300mV
pp
DC electrode offset and 400mV
pp
common-mode (CM)
interference, a DC-servo loop (DSL) combined with a common-
mode feedforward (CMFF) loop is employed. A buffer-assisted
scheme boosts the IA’s input impedance by 7x to 140MΩ at 10Hz.
To improve the BioZ sensitivity, the CG utilizes dynamic element
matching to reduce the 1/f noise of the output current, leading to
35mΩ/√Hz BioZ sensitivity down to 1Hz. The ADC shows a 9.7b
ENOB when sampled at 20ksps. The total power consumption of
the AFE is 3.8μW.
Keywords—ECG; bio-impedance; instrumentation amplifier;
low noise current source; low supply
I. INTRODUCTION
Chronic cardiovascular-respiratory diseases, like congestive
heart failure (CHF) and obstructive sleep apnea (OSA), require
long-term, continuous and comfortable monitoring of ECG and
bio-impedance (BioZ) to detect abnormal heart rate, respiration
and body fluid volume. For miniaturized, lightweight and low-
cost disposable patches, alternative power sources such as
organic paper batteries, 3D printed batteries or thermal energy
harvesters are more interesting than bulky Lithium-ion cells.
However, these promising batteries usually have a low output
voltage, which would require circuits also operating at sub-volt
supplies to avoid excessive power management losses of boost
converters. Furthermore, a low supply AFE enables better co-
integration with digital cells to facilitate power-efficient and
on-the-node signal processing.
State-of-the-art IC solutions do not meet these requirements
at the same time. Ultra-low power ICs [1]-[4] operating at 0.5-
0.6V do not support BioZ and they are compromising on noise
performance (i.e. [2][3] don’t meet the noise requirement of
<30μV
pp
defined in ANSI/AAMI/IEC60601-2-47), while high-
performance multimodal ECG/BioZ ICs
[5][6] typically have
1.2-1.8V supplies and consume more power (>50μW/channel).
IA
0.6V
ECG
BioZ
SAR
ADC
CS
Bias
LPF
Digital
Filter
20ksps
Fig. 1. IC block diagram and simultaneous ECG and BioZ measurement with
one single amplifier
This work presents a 0.6V 3.8μW AFE (Fig. 1) including
an instrumentation amplifier (IA), a BioZ current source (CS)
and a SAR ADC to facilitate simultaneous monitoring of ECG
and BioZ. An orthogonal frequency modulation scheme [7]
enables power-efficient ECG and BioZ measurements with a
single IA. To cope with large electrode-offsets and common-
mode mains interference on the 0.6V low supply, a DC-servo
loop (DSL) combined with a common-mode feedforward
(CMFF) path is proposed. This allows the IA to tolerate up to
300mV DC electrode offset (DEO) and 400mV
pp
input CM
fluctuation, respectively. BioZ measurement is enabled by a
wide-swing and low-noise current generator equipped with
regulated current mirrors and dynamic element matching
(DEM).
II. C
IRCUIT IMPLEMENTATION
A. Orthognal Frequency Modulation
C1
C1
C1
C1
fc=4kHz
f
BioZ1=
31kHz
f
BioZ2
=1kHz
fc=4kHz
C2
C2
Fig. 2. Simultaneous ECG and BioZ measurement with one single amplifier
While traditional multimodal IC implementations rely on
dedicated ECG and BioZ readouts, this work proposes a single
amplifier-based ECG and BioZ readout (Fig. 2). A capacitively
coupled IA (CCIA) concurrently measure both signals from the
same electrodes by making use of signal properties: the BioZ is
measured by injecting an AC current above 1kHz. Hence, the
ECG and modulated BioZ signals appear in different frequency
bands before entering the IA. However, medically relevant
BioZ spans from 1kHz-128kHz, this would impose strict BW
requirements and high power on the readout [6]. To overcome
this issue, the modulated BioZ signal is firstly down-converted
to 1kHz (Fig. 2) before the CCIA and then demodulated further
to DC at the output of the CCIA. The ECG signal is chopped at
4kHz, such that the fundamental and harmonic components of
ECG and BioZ signals during amplification are located at
different frequency bins in an orthogonal manner. At channel
outputs, both ECG and BioZ signals are modulated back to the
baseband respectively without interfering each other. The SAR
ADC is oversampled at 20ksps to avoid folding of noise and
residual harmonics. This also relaxes the design of anti-aliasing
LPF in terms of its bandwidth and order. Sharing one CCIA for
both ECG and BioZ channels improves power efficiency and
reduces chip area, while the orthogonal frequency modulation
ensures more than 60dB signal isolation between channels.
B. Instrumenation Amplifier (CCIA)
The biggest design challenge for a 0.6V bio-amplifier is to
ensure almost rail-to-rail input and output dynamic range in the
presence of large external signals (300mV DEO, baseline drift
and mains CM variations). This design utilizes the CCIA (Fig.
3) because it requires near-zero voltage swing at the virtual
ground. To improve power-efficiency of the CCIA, the core
amplifier is based on an inverter-based input stage [3] (Fig. 4)
and a class A/AB output stage with switched-capacitor CMFB.
Since chopping at the virtual ground node of a CCIA increases
the noise, the choppers are implemented around the capacitive
feedback network. However, this modulates the DC signals at
the same time, and coupling capacitors C
1
would fail to reject
the DEO. To solve this issue, a DSL is provided to compensate
the DEO (Fig. 3). The DC voltage at ECG output is tracked by
a Gm-C integrator and a compensation current at fc=4kHz is
fed back to the virtual ground via C
fb
to null the DEO current.
The Gm has a complementary input (Fig. 4) to support CCIA’s
rail-to-rail output swing, and is chopped to reduces residual 1/f
noise.
Fig. 3. Block diagram of the 0.6V ECG/BioZ instrumentation amplifier
1uA
vbp
vin
vip
vcmfb1
vnp
vnn
vnn
vnp
vnpvnn
vcmfb2
voutpvoutn
50nA
3:2 2:3
50nA
1:1 1:1
vcmfb3
4:1
1:4
vbp vbp
vbp
vbn
casn
casp
vbn
400nA
250nA
250nA
250nA
400nA
vcmfb3
voutn
voutp
voutpvoutn
vbn
50nA
vcmff
vcmff
p1
p1 p2
p2
p1
p2
p2 p1
SC-CMFB
CMFB
IA input stage
IA output stage
Gm stage of the DSL
Fig. 4. Schematic of IA including inverter-based input, class A/AB output, Gm
stage for DSL and CMFF
Although inverter-based input stages are attractive for low-
power, they suffer from distortion with the presence of a large
CM input signal. This is problematic for wearable biomedical
applications where CM interference can be significant. Hence,
a CMFF loop like [8] is used to reduce the CM swing at the
virtual ground for improved linearity (Fig. 5). However, in this
work, the input CM is fed forward to the virtual ground via the
DSL’s SC-CMFB reference (i.e., vcmff in Fig. 4) and C
fb
,
instead of adding another feedback loop [8]. Hence, the noise
due to multiple feedback paths is reduced.
C1=4pF
fc=4kHz
Cf1=2pF
Cf2=4pF
Gm
fc=4kHz
fc=4kHz
vcmff
ECG input
ECG output
VCM
VCM
VCM
VCM
VCM
C1=4pF
Cfb=4pF
Cfb=4pF
VS≈0
VS≈0
VDEO
VDEO
VDEO≈0
2
=
2
1
+
2
+
1
2
Fig. 5. CMFF combined with DC servo for noise reduction
The chopping CCIAs suffer from limited input impedance
formed by SC resistors. Prior work employed positive feedback
loops to boost input impedance [3][9] but they may suffer from
instability and the practical boosting factor heavily depends on
parasitic capacitance. In this work, the input impedance of the
CCIA is boosted by two pre-charging buffers placed after the
input chopper (Fig. 6), this is similar to [8] but with a different
clocking scheme. The buffers are periodically connected to the
signal path for 15.625µs whenever the chopping clock switches.
Hence, the spike current to charge C
1
is provided by the buffers,
instead of ECG source input. This reduces the net current draw
from the source and thus improves the input impedance over
the entire bandwidth. In addition, this approach eliminates the
instability risk. Thanks to the duty-cycling buffers, their noise
contribution is negligible.
x1
x1
C1
C1
C1=4pF
C1=4pF
C2
C2
fc=4kHz
fi
1
fi
1
fi
2
fi
2
C
CM
=2pF
Cf=4pF
Cfb=4pF
10uF Gm
fbi=f
BioZ
-1kHz
fbo=1kHz
fc=4kHz
fc=4kHz
fc=4kHz
fc=4kHz
DC servo and CMFF
Impedance
Boosting
vcmff
ECG input
BioZ input BioZ output
ECG output
BioZ channel
x1
x1
C1
C1
C2
C2
fc=4kHz
fi
1
fi
1
fi
2
fi
2
fc=4kHz
Impedance
Boosting
ECG input
ECG output
fc=4kHz
fi
1
fi
2
16.265
µ
s
C
p
C
p
C
CM
C
CM
=
//
1
//
1
4
Fig. 6. Input impedance boosting with auxiliary buffers
C. BioZ Current Generator
The BioZ current source (CS) is shown in Fig. 7. It has an
output magnitude of 5μ-100μA and an output frequency of 4k-
128kHz. Since most of BioZ activities (e.g., respiration, body
fluid volume) are below 10Hz, a main design challenge is to
reduce the CG’s 1/f noise for improved sensitivity. Apart from
noise, achieving a large compliance range under 0.6V supply is
also important. This ensures that the CS remains operational
when considering voltage drop over the electrode impedance,
which is typically larger than the BioZ.
casp
casn
5uA
5uA
Vdd
Magnitude
Control
I
OUT
=5-100uA
4k-128kHz
Electrode
Impedance
BioZ
I
OUT
f
DEM
/(N+1)
freq
in,out
2
Fig. 7. Block diagram of the 0.6V BioZ current source
To meet these requirements, the CS has dynamic element
matching (DEM) between all unit current mirrors to modulate
their 1/f noise to f
DEM
/(N+1) (Fig.7), where N is the current
amplification factor. In this work, f
DEM
is selected to be 16kHz
or 32kHz. The CG utilizes active cascode current mirrors to
improve the voltage compliance, where two OTAs regulate the
V
ds
of all mirror transistors and ensure their matching in triode
region. The compliance voltage of the CG is 400mV
pp
(~67%
of V
dd
) at the maximum current of 100µA
pk
The OTAs are also
chopped at 8kHz to reduce their 1/f noise.
III. M
EASUREMENT RESULTS
The IC is implemented in TSMC 40nm CMOS and the chip
area is 1mm
2
(Fig. 8). The readout consumes 6.3µA from 0.6V
with the CS supporting current levels from 10µA
pp
to 200µA
pp
.
Fig. 8. Chip photograph
In Fig. 9, the ECG channel has a passband voltage gain of
30dB and an input referred noise of 145nV/sqrt(Hz), where
BioZ channel is also enabled. The ECG/BioZ channel crosstalk
is less than -60dB in a 400Hz bandwidth. With the help of two
pre-charging buffers, the CCIA’s input impedance is improved
by 7x, from 20MΩ to 140MΩ at 10Hz. The CCIA shows its
robustness to the DEO (Fig. 10). When 300mV DEO is applied,
the CCIA still show less than 200nV/sqrt(Hz) input noise and a
flat gain of 30dB.
Fig. 9. ECG channel measurement results
Fig. 10. ECG noise and gain versus DEO
The BioZ channel shows 35mΩ/sqrt(Hz) sensitivity when a
20µA
pp
, 32kHz output current is applied to a 100Ω test resistor.
This sensitivity includes the both the noise of the CCIA and the
CS. Another BioZ test with resistors of 10-200Ω shows good
linearity and matching with respect to theoretical numbers.
The 13b SAR ADC achieves a 9.7b ENOB at 20ksps while
consuming 400nA from 0.6V. These correspond to a FoM of
15fJ/conversion. The ADC power dissipation linearly increase
with its sampling rate, the current is 10µA at 400ksps.
ECG gain measured at analog output
ECG channel noise
G=30dB, fc=4kHz
Pre-charging buffer on
Pre-charging buffer off
Input impedance is boosted
by 7x at 10Hz
Vin,
ECG
=10mV
pp
, G=30dB
Input noise measured at 20Hz
Voltage gain measured at 20Hz
Fig. 11. BioZ channel noise and linearity
Fig. 12. ADC output spectrum, ENOB, and power
Fig. 13 shows the simultaneous ECG and BioZ recordings
obtained from the same sensing electrodes on the chest. ECG
signals and respiratory impedance change are clearly visible.
Fig. 13. Simultaneous ECG and BioZ recording from the same electrodes
Table I compares this work with prior-art low voltage ECG
ICs and multimodal ECG/BioZ ICs. This work advances the
existing 0.5-0.6V ICs [1]-[4] in terms of noise, CMRR, input
impedance and input CM range. Compared to multimodal ECG
and BioZ ICs [5][6], this work achieves competitive accuracy
but with 2x lower supply voltage and 15x lower power.
IV. C
ONCLUSIONS
This work presents the first 0.6V IC for simultaneous ECG
and BioZ recording. Both signals are amplified with one single
IA through orthogonal frequency modulation. The combination
of a DSL and a CMFF improves the CCIA’s tolerance to DEO
and CM interference without adding noise. Lastly, a DEM- and
active cascode-based CG realizes both low noise and improved
voltage compliance.
R
EFERENCES
[1] M. Yip, J. L. Bohorquez et al., "A 0.6V 2.9µW mixed-signal front-end
for ECG monitoring," IEEE Symposium on VLSI Circuits, pp. 66-67,
June. 2012.
[2] R. Mohan, S. Zaliasl et al., "A 0.6-V, 0.015-mm2, time-based ECG
readout for ambulatory applications in 40-nm CMOS," IEEE J. Solid-
State Circuits, pp. 298-308, Jan. 2017.
[3] P. Harpe, H. Gao et al., “A 0.20 mm2 3nW signal acquisition IC for
miniature sensor nodes in 65 nm CMOS”. IEEE J. Solid-State Circuits,
pp. 240-248 Jan. 2016.
[4] R. Muller, S. Gambini et al., "A 0.013mm
2
,5µW, DC-coupled neural
signal acquisition IC with 0.5V supply," IEEE J. Solid-State Circuits,
pp. 232-243, Jan. 2012.
[5] N. Van Helleputte, M. Konijnenburg et al., “A 345µW multi-sensor
biomedical SoC with bio-impedance, 3-channel ECG, motion artifact
reduction, and integrated DSP," IEEE J. Solid-State Circuits, pp. 230-
244, Jan. 2015.
[6] J. Xu, P. Harpe et al., “A low power configurable bio-impedance
spectroscopy (BIS) ASIC with simultaneous ECG and respiration
recording functionality,” Proc. of ESSCIRC, pp. 396-399, Sept. 2015.
[7] Y. L. Tsai, F. W. Lee, T. Y. Chen and T. H. Lin, "A 2-channel −83.2dB
crosstalk 0.061mm
2
CCIA with an orthogonal frequency chopping
technique," Digest of ISSCC, pp. 1-3, Feb. 2015.
[8] H. Chandrakumar, D. Markovic., “A 2.8µW, 80mV
pp
linear-input-range,
1.6GΩ input impedance, bio-signal chopper amplifier tolerant to
common-mode interferers up to 650mV
pp
,” Digest of ISSCC, pp. 448-
449, Feb. 2017.
[9] Q. Fan, F. Sebastiano et al., "A 1.8μW 60nV/sqrt(Hz) capacitively-
coupled chopper instrumentation amplifier in 65nm CMOS for wireless
sensor nodes," IEEE J. Solid-State Circuits, pp. 1534-1543, July. 2011.
IBioZ=20µApp, fBioZ
=32kHz
BioZ channel sensitivity
Rs=100Ω, I
out
=20uApp@32kHz
Input signal at 20Hz
The subject was
holding his breath
Respiratory impedance change
Respiratory impedance change
TABLE I:
Parameters
[1]
[2]
[3]
[4]
[8]
[5]
[6]
This Work
Acquisition
modes
ECG
ECG
ECG
LFP
ECG
ECG, BioZ
(two IAs)
ECG, BioZ
(single IA)
ECG, BioZ
(single IA)
Technology
180nm
40nm
65nm
65nm
40nm
180nm
180nm
40nm
Supply voltage
0.6V
0.6V
0.6V
0.5V
1.2V
1.2V
1.8V
0.6V
Max. EDO
rail-to-rail
150mV
rail-to-rail
50mV
N/A
400mV
rail-to-rail
300mV
Input CM range
N/A
N/A
N/A
N/A
N/A
650mV
pp
N/A
400mV
pp
Input noise
(150Hz BW)
3.44µV
rms
7.8µV
rms
26µV
rms
4.3µV
rms
(300Hz BW)
1.8µV
rms
0.61µV
rms
0.6µV
rms
1.85µV
rms
Gain
34.5dB
N/A
32dB
32dB
25.7dB
28/36dB
4/16/56dB
20/30dB
Input Impedance
N/A
50MΩ
N/A
N/A
1.6GΩ@1Hz
500MΩ@50Hz
10MΩ
140MΩ@10Hz
CMRR
70dB
60dB
60dB
75dB
N/A
110dB
60dB
87dB
Power
(excl.CS)
1.15µW
(ECG)
3.3µW
(ECG)
0.003µW
(ECG)
5.04µW
(LFP+Spike)
2.8µW
(ECG)
56µW(ECG)
58µW(BioZ)
155µW
(ECG+BioZ)
3.8µW
(ECG+BioZ)
BioZ
sensitivity
--
--
--
--
--
9.8mΩ/√Hz
(excl.CS noise)
100mΩ
pp
35mΩ/√Hz
ADC ENOB
9b
N/A
9.2b
9b
--
13.5b
10.5b
9.7b