A Electrode-Tissue Impedance Measurement System
with Low-Power Compressive Sampling Time-Based
A/D Converter
Group 4: Shuxun Cao, Jun-Yang Lei, Tso-Wei Li
Abstract—In this paper, we proposed an electrode-tissue
impedance (ETI) measurement method incorporating a low-
power compressive sampling, time-based analog to digital
converter. Electrode-tissue impedance monitoring is an
important part of the Electrocardiogram (ECG) recoding system
since it could be used to suppress erroneous ECG signals due to
the motion artifact in ECG recording, ensuring correctness of the
whole system. In our system, we incorporate an ADC with a
time-based processing and compressive sampling techniques,
which lead to a more energy-saving and efficient ADCs for this
biomedical system.
Keywords—Electrocardiogram (ECG), instrumentation
amplifier, motion artifact reduction, Analog-to-Digital Conversion,
electrode-tissue impedance.
I. I
NTRODUCTION
In modern society, cardiovascular diseases and heart failure
are the major causes of death in many countries. Thus, there
has been growing needs for devices that could constantly
monitoring electrocardiogram (ECG) signals (Fig.1) through
portable devices [1][2] in recent years. However, the major
technical challenge with ECG measuring for mobile and
portable devices is dealing with high level of noise introduced
by motion artifacts caused by patients’ motion [1]-[5]. For the
mobile ECG detection devices, identifying artifacts is crucial
for the correct operation of the devices as they may lead to
false alarms and wrong event detections. If the motion artifact
could be filtered out properly, it could extend the usage of the
mobile ECG recording device, ensuring its precise operation
of the device for patients using them in their daily life.
Some of the methods are proposed to reduce the noise from
motion artifacts [15]-[18]. However, those methods mostly
required extra sensors to collect data and hence consume more
power for the overall system. It has been studied that a strong
correlation exists between motion artifacts (MA) and
electrode-tissue-impedance (ETI) [2][6]. This strong
correlation provides opportunities for us to measure the ETI at
the analog domain without extra sensors implemented and
eliminate the effect of MA from ECG signals using signal-
processing algorithm.
Thus, in this work, we propose an Analog Front End that is
Fig. 1 Electrocardiogram (ECG) recording for diagnosing cardiovascular
diseases[1].
capable of acquiring the electrode-tissue impedance by
processing the real and imaginary components of the ETI.
which are then being digitalized and being used as a reference
signal for post processing by digital-signal processing unit.
From the constant monitoring of the electrode-tissue
impedance, the motion artifacts could severely corrupt the
ECG signal could be eliminated.
To realize a portable and wireless ECG recording system,
low-power consumption for each building block of the system
is required. In this work, we incorporate a low-power
compressive sampling time-based analog-to-digital converter
[10]. This A/D converter is one of the first ADCs that combine
time-based analog-to-digital conversion techniques with
compressive sampling. In this ADC, a continuous-time
comparator compares the input to a periodic ramp, to convert
the input signal information to a time-domain representation.
The time domain pulses are then converting to digital domain
by a two-step time-to-digital converter (TDC). With the use of
a two-step TDC, the ADC achieves both high resolution and
high dynamic range, along with low-power consumption.
Another way to obtain an improvement in the power
efficiency of an ADC is to reduce the sampling rate since
power consumption is proportional to sampling frequency.
This is achieved by employing random sampling techniques
that exploit the redundancy (i.e., compressibility or sparsity)
of the input signal to reduce the sampling rates to below the
Nyquist rate. We implement random sampling by introducing
randomness into the reference ramp signal used by the PPM
ADC. The proposed random PPM ADC lies at the intersection
of time-based analog-to-digital conversion and compressive
sampling to improve energy efficiency in both ways.