IEEE Solid-State Circuits Magazine - Fall 2017 - 93

Bias
Voltage

∆V1
E1

2.1 V

1.5 V

∆V3 ∆V1, ∆V2
1.8 V

1.5 V
1.2 V

∆V2

1.5 V

0.9 V

∆V3

+ 1.5 V -
V

Skin

(a)

Gain
Rc

ZE1

Ref. Electrode E2

ZE2

RS + Rin

Rin

Frequency

RS + RC + Rin

Rin

(b)

Meas. Electrode

Rin

Gain from
Subject to Amp

Zin

V+

V-

RCCC

RC ||(RS + Rin)CC

VCM = Id × ZE3
V+ = VCM ×
V- = VCM ×

Zin

E3
ZE3

Bias Electrode

CMRR =

ZIN
ZIN + ZE1
ZIN
ZIN + ZE2

V+ - V-
VCM

ZE1 - ZE2
ZIN

(c)

VH
Skin

The second challenge is about the
large impedance of the electrode as
shown in Figure 3(b). For example,
a dry electrode has large Rc, as high
as several MXs. Such large electrode
impedance may attenuate the biopotential signal before amplification
due to limited input impedance (R in)
of the amplifier. More seriously, a
low-frequency biopotential signal near
dc is prone to be distorted since
the capacitance Cc reduces the electrode impedance as the frequency
increases. Unless R in is high enough,
the phase shift of the low-frequency
signal can be pronounced [4].
The third challenge [Figure 3(c)]
is associated with the impedance
mismatch between electrodes E1 and
E2. Even if the CMRR of the amplifier itself is infinite, the VCM on the
subject can be converted into the
differential signal provided that the
impedance mismatch between E1
and E2 (Z E1-Z E2) is comparable to the
input impedance (Z IN) of the amplifier. This is another important reason why the (common-mode) input
impedance of the amplifier must be
high enough.
The last, but not least, motion artifact is emphasized in Figure 3(d).
In the steady state of the electrode
interface, the electrolyte balances the
charge at the electrode-tissue interface. Once the subject moves, the
charge balance is disturbed, translating the change of the charge into
the contact impedance Z E and halfcell potential VH . The change of VH
directly affects the input voltage signal, and the change of Z E modulates
the VH building-up motion potential
signal, leading to the baseline signal
fluctuation. Especially when it comes
to the dry electrode, the motion artifact is much more severe since there
is lack of electrolyte at the electrode interface.
In summary, it is important to
understand 1) the existence of the
large common-mode signal on the
subject through the displacement current and bias electrode, 2) the large
dc offset signal due to the electrode
polarization, 3) the requirements of

∆VH

ZE + ∆ZE
Iin
ZE

ZIN
Gain

ZIN
∆Vout = Gain × (∆VH + VH × ∆ZE /ZIN + ∆ZE × Iin)
ac Coupling
or Large Zin

Minimize
Input Current

(d)
FIGURE 3: Electrode interface challenges: (a) dc polarization voltage, (b) amplitude attenuation, (c) CMRR degradation due to electrode impedance mismatch, and (d) motion artifact.

the high-input impedance amplifier
to support high electrode impedance
and its mismatch, and 4) the motion
artifact due to the change of the electrode interface in motion. With these
challenges in mind, the readout circuit design must focus on achieving
high CMRR, large dc filtering range,

high input impedance, low noise with
low power consumption. The circuit
designer has to be able to trade those
performance parameters off with
res pect to the application requirements. According to the applications,
the minimum requirements for the
readout circuits can vary as shown in

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