IEEE Solid-State Circuits Magazine - Fall 2017 - 91

Electrodes are one of the most
widely utilized transducers for physiological signal measurement. They
convert ionic information inside the
human body into electrical voltage signals. In general, a dedicated readout
circuit is required to amplify weak electrical signals from electrodes buried
by unwanted noise signals. This article
introduces fundamentals on physiological signal measurements using
electrodes helping readers to understand instrumentation challenges.

Understanding the Challenges
of Electrode Interfaces
Electrode Interfaces
Figure 1 shows a biopotential signals acquisition system with an electrode interface. Electrocardiogram
(ECG), electromyograph (EMG), and
electroencephalogram (EEG) are the
most widely monitored physiological signals that represent activity
of the hear t, muscle, a nd brain,
respectively. In general, the signal
(bandwidth and amplitude) characteristics of the ECG, EMG, and
E EG a r e in t he r a nge of 0.05 -
250 Hz/0.5-4 mV, 20-1,000 Hz/0.1-
5 mV, a n d 0.5-150 Hz/5-300 nV ,

respectively [1]. Notice that their
bandwidths are located in the low frequency under 1 kHz, and the signal
strength is extremely small. Given
the signal bandwidth and amplitude,
a good circuit designer needs to design a biopotential readout circuit tolerable to a large degree of unwanted
noise signal such as 50/60-Hz interference, flicker noise, and dc offset of
the amplifier. Biopotential electrodes,
as the first component of a signal
acquisition chain, play an important
role in biopotential measurements as
the characteristics of the electrode-tissue interface can be a limiting factor
of system performance. The proper
topology of a readout circuit and
the minimum required performance,
such as, noise, common-mode rejection ratio (CMRR), input impedance,
and power consumption are usually
derived from this interface.
The electrode-tissue interface
can be modeled w ith electrodetissue impedance and polarization
voltage. C elec and R elec represent the
impedance related with the electrodeelectrolyte interface, and R Gel is
the resistance of the electrolyte (gel)
solution. The half-cell potential, VH,
is due to the built-up charge gradients

EEG

at the electrode-electrolyte interface
[2]. The impedance with C epi, R epi,
and R D is the representation of the
skin and its equivalent circuit model.
The circuit model parameters largely depend on the electrode material,
area, skin condition, invasiveness,
and pressure of electrode contact. A
typical wet electrode with Ag/AgCl
and hydrogel is widely used in clinical practice due to its low polarization voltage (0.22 V), low impedance
(51 kΩ in parallel to 47 nF), and low
baseline drift (0.13 mV at room temperature). Dry contact electrodes,
however, eliminate the use of gel
at the cost of a significant higher
impedance (high resistance and small
capacitance), which requires further
efforts for amplifier design.

Electrode Configuration
Practical electrode applications for
the biopotential signal measurements are shown in Figure 2. To pick
up signals from the body, a pair of electrodes, E1 and E2, is used. Z E1 and Z E2
represent contact impedance of the
electrodes E1 and E2, and V1 and V2
are voltage-converted signals through
the electrodes. Suppose that a differential amplifier powered by a battery

Electrode System

ECG

Celec

Relec

Electrode
Gel
Stratum Corneum
Epidermis

EMG

EEG: 5-300 µ V
0.5-150 Hz
ECG: 0.5-4 mV
0.05-250 Hz
EMG: 0.1-5 mV
20-1,000 Hz

Subcutaneous
Layers
Dermis

RGel
Half-Cell
Potential

AMP

Cepi

Repi

ex. Ag/AgCl
Relec = 51 kΩ /Celec = 47 nF/VH = 0.22 V

FIGURE 1: A biopotential signals acquisition system with an electrode interface.

IEEE SOLID-STATE CIRCUITS MAGAZINE

FA L L 2 0 17

Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Fall 2017