Instrumentation & Measurement Magazine 24-2 - 38

is an electrical isolation module, which provides electrical
safety from any electrical surges or shorts that may occur in
the system.

Contaminants

Fig. 1. Annotated ECG, with the time between successive heartbeats labeled
as the 'R-R Interval.'

The P-wave is associated with the contraction of the atria of
the heart, the QRS complex with the contraction of the heart
ventricles, and the T-wave with the repolarization of the ventricles. The peak amplitude of the ECG is on the order of a few
millivolts, and the bandwidth for clinical ECG measurements
ranges from 0.05 Hz to 150 Hz [11].
The ECG can be recorded noninvasively using electrodes
placed on the surface of the skin. Electrodes are connected to
an analog frontend that typically has five main modules (Fig.
2). The first module is the bio-amplifier, which is a precision
instrumentation amplifier, with a high input impedance (>10
MΩ) and a high common-mode rejection ratio (>90 dB). Additional amplification stages may be included in the analog
frontend. The second module is the right leg driver, which estimates the common-mode interference on the body and uses
negative feedback to reduce common-mode interference actively. The third module is the filter bank, which attenuates
undesired frequency components (e.g., low-frequency baseline wandering, harmonic frequencies associated with power
line interference). The fourth module is the analog-to-digital
converter (ADC), which digitizes the ECG. The final module

Biosignal contaminants can be divided into three types: measurement; instrumentation; and interference [12]. Table 1 lists
examples of ECG contaminants, and Fig. 3 illustrates the appearance of a few of these contaminants.
Motion artifact and skin stretch reflex contaminants are
caused by changes in the electrode-skin interface due to electrode motion and changes in the skin geometry. They are
low-frequency contaminants, with much of the artifact below
10 Hz. Baseline wandering is a very low-frequency contaminant (< 0.6 Hz) caused by slow changes in the electrode-skin
interface (e.g., due to perspiration). A high-pass filter can remove baseline wandering [11]. Motion artifact and skin stretch
reflex contaminants are harder to remove, as they overlap
with the ECG in both the time and frequency domains. Poor
electrode contact can occur when an electrode is loosely attached or when there is inadequate skin preparation before
the electrode is attached. Poor electrode contact results in
high electrode-skin impedance, which decreases the recorded
amplitude of the signal, introduces large wandering signal deviations, and potentially leads to intermittent loss of the ECG
during recording (e.g., electrode lift). Many ECG systems have
lead-off detection based on monitoring source impedance.
Amplifier saturation and ADC over-ranging result from
the analog signal amplitude exceeding the operational limits
of the modules and lead to clipping of the signal. Quantization noise is introduced during analog-to-digital conversion,
as continuous amplitude values are approximated by a set of
a finite set of values. With uniform quantization, the signal to
quantization noise ratio is proportional to 6N dB, where N is
the number of quantization bits. If the number of bits is low,
the digitized ECG may not represent the analog signal, with
noticeable distortions, obscuring important features. Typical
ECG recorders use 10 to 12 bits, a resolution of 9.76 to 2.44 mV,
assuming a full scale of 10 mV.

Fig. 2. Block diagram of an analog frontend for an ECG recorder.
38	

IEEE Instrumentation & Measurement Magazine	

April 2021



Instrumentation & Measurement Magazine 24-2

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