IEEE Solid-State Circuits Magazine - Fall 2017 - 92

Id = 1µA

Displacement Current (Id)
from Mains

E1

ZE1

E2

ZE2

V1
V2 +

+

+

-

3V
-

VCM = Id × Zg
= 26 V
Zg = 100 pF

Ground
Coupling (Zg) -

Meas.
Electrode E1
Ref./Bias E2
Electrode

V1

ZE1
ZE2
Id

-

ZE1
ZE2
ZE3

V1

V

Sensitive to
VBIAS

Battery GND

VCM = VDM
= Id × ZE

Earth GND
(b)

+
-

V2

Vout = AV × (V1-VBIAS)

V VBIAS

(a)
Meas.
Electrode E1
Ref. E2
Electrode
E3
Bias
Electrode

+

VBIAS

Id
(c)

Vout = AV × (V1-V2)
Insensitive to
VBIAS
VCM = Id × ZE
VDM = V1 - V2

Meas.
Electrode E1
Ref.
Electrode E2

ZE1
ZE2

V1

+
-

V2
(V1+V2)/2

RLD E3
Electrode

ZE3

A
(d)

VCM = Id × ZE /(1+A)
VDM = V1 - V2

FIGURE 2: Electrode usage for biopotential signal measurements. (a) A common-mode signal, (b) unipolar electrode readout, and (c) bipolar
electrode readout, and (d) common-mode feedback electrode readout.

is used to amplify signals (V1-V2) .
The question is, "Can we measure
reliable biopotential signal with
such electrode configuration shown
in Figure 2(a)?" Before answering, we
need to figure out the common-mode
potential (VCM) of the human body.
The human subject is always exposed
to 50/60-Hz environments, which
means that there is a displacement
current (I d) coupling with a few nA
to several nA, depending on the surroundings. For instance, the body is
coupled (100 pF) to the earth ground,
where 1 nA displacement currents
are flowing. This implies the value of
VCM is as high as 26 V from the earth
ground. Furthermore, the VCM might
change with respect to the coupling
strength between earth ground and
battery ground, resulting in the readout amplifier easily missing the signal
due to its limited dynamic range.
To secure the biopotential signal
always within the dynamic range of
the readout amplifier, one must form
a closed-loop circuit to the subject by
proper subject biasing. Exerting the

92

FA L L 2 0 17

well-defined bias voltage to the subject not only provides a low impedance path between the subject and the
battery ground but also eventually
lowers the common-mode voltage of
the body. Figure 2(b) shows a unipolar electrode readout configuration,
where E1 is a measurement electrode
and E2 is a bias electrode. Therefore,
any signals on E1 will be amplified. It
is, however, susceptible to the noise
and interference since the common-mode and differential-mode
voltages are identical. Figure 2(c)
impr oves the readout configuration by separating the bias electrode
using another electrode, E3. In this
bipolar electrode readout, the VCM is
determined by the impedance of E3,
whereas the common-mode noise
will be mitigated by the differential measurement from E1 and E2.
Common-mode feedback shown in
Figure 2(d) can be applied to further
reduce the level of VCM from E3. The
principle is that common-mode signals from E1 and E2 are extracted by
averaging V1 and V2. With a gain of

IEEE SOLID-STATE CIRCUITS MAGAZINE

A, the effective impedance of E3 significantly gets reduced. In practical ECG
recording, this scheme is known as
driven-right-leg to suppress 50/60 Hz
interference [3].

Challenges of Biopotential Readout
As previously mentioned, electrodes
may develop dc polarization voltage
across the electrode interface. Sometimes, this polarization voltage in the
electrode system brings up significant challenge. Figure 3(a) addresses
the dc polarization issue, assuming
that each electrode has 300-mV polarization voltage. Suppose that subject
is biased with 1.5 V, and the bipolar
electrode readout is configured. In the
worst case, the bias voltage through
E3 can be either 1.8 or 1.2 V. And E1
and E2 may also introduce polarization voltage on top of this. This may
change the common-mode signal on
E1 or E2 either down to 0.9  V or up
to 2.1 V. Considering that biopotential
signals of near dc are only a few nVs,
a huge dynamic range must be guaranteed by readout circuits.
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