Selection of our books indexed in the Book Citation Index in Web of Science™ Core Collection (bkci)

Figure 37. A simplest single-input amplifier
Figure 38. A differential amplifier
There is also capacitor introduced in figure 37 in series with the input signal. Capacitor 
C
blocks
any DC (direct current) signal by acting as a high-pass filter with the resistor 
R
1
. This function
is usually referred as AC (alternative current) coupling. AC coupling will also cause some
attenuation of the signal which may be important. We could determine the attenuation of
signal by the following equations:
u
out
=u
−
=u
+
,
u
in
R
1
+
1
jωC
=
u
+
R
1
, then
u
out
u
in
=
R
1
R
1
+
1
jωC
=
R
1
(
R
1
+ j
/
ωC
)
R
1
2
+ 1
/
ω
2
C
2
|
U
out
U
in
|
=
1
1 + 1
/(
ω
2
R
1
2
C
2
)
=
1
1 + ω
0
2
/
ω
2
, where ω
0
=1
/
R
1
C.
Advances in Bioengineering
222

---

If capacitor C is 1μF and resistor 
R
1
is 1MΩ, then the attenuation of a 1Hz signal will be 1.25%.
This is perhaps a significant attenuation for ECG which has considerable energy at 1Hz.
Unfortunately, even with capacitor C added, this type of amplifier is not suitable for recording
small bioelectric signal because of interference from external electric fields. An electric
electrode has to be connected to the amplifier via a wire and this wire is exposed to interfering
signals. However, the interference will only appear on the input wire to the amplifier and not
on the “ground” wire which is held at zero potential. An elegant solution to this problem is to
utilize differential amplifier as shown in figure 38. The input of the type of amplifier has three
connections marked ‘+’, ‘-’ and ‘ground’.
• Differential amplifier
In figure 38, the signal which we wish to record is connected between the ‘+’ and ‘-’ points.
Now both inputs are exposed to any external interfering electric fields so that the difference
between the ‘+’ and ‘-’ input will be zero. This will not be quite true because the electric fields
experienced by the two input wires may not be exactly the same, but if the wires are run close
together then the difference will be small. Differential amplifier is not perfect in that even with
the same signal applied to both inputs, with respect to ground; a small output signal can
appear. This imperfection is specified by the common mode rejection ratio or CMMR. An ideal
differential amplifier has zero output when identical signals are applied to the two ‘+’ and ‘-’
inputs. CMMR could be defined as the following equation:
CMMR =20log
signal gain
common −mode gain
Where, signal and common-mode gains are given by
signal gain =
U
out
U
in
=
U
out
U
+
−U
−
common −mode gain =
U
out
U
cm
=
U
out
(
U
a
+ U
b
) /
2
In practice, common-mode voltage 
U
cm
can be as large as 100mV or even more. In order to
reject this signal and record a signal V
in
as small as 100μV, a high CMMR is required. If we
wish the interfering signal to be reduced to only 1% of output voltage then
required −signal gain =
U
out
U
in
=
U
out
U
+
−U
−
=
U
out
100μV
required −CM gain =
U
out
U
cm
=
U
out
1%U
in
=
U
out
/
100
100mV
Hence, the required CMMR could be given as:
Biomedical Sensor, Device and Measurement Systems
http://dx.doi.org/10.5772/59941
223

---

CMMR =20log
U
out
/
0.1mV
U
out
/
100
2
mV
=100dB
In fact, it’s not always easy to achieve a CMMR of 100dB. As we have known, electrode source
impedances have a very significant effect on CMMR and hence electrode impedance affects
noise rejection.
Of course, in detecting biosignals, the AC coupling shown in figure 37 and figure 38 degrades
the performance of the amplifiers. If the input impedance and bias current of amplifiers is
sufficiently high, then they can be connected directly to the input electrodes, without produc‐
ing electrode polarization. Furthermore, DC offset will occur from the electrode contact
potentials, but if the amplifier gain is low (<10) DC offset will be not a significant problem. The
offset can be removed by AC coupling at later stage.
However, there are some safety arguments against the use of DC coupling. If a fault arises in
the operational amplifier, then it’s possible for the power supply to be directly connected to
the patient and so give rise to a hazard. DC currents will cause electrolysis and result in tissue
necrosis. AC coupling could avoid this problem and is often used. Nonetheless DC coupling
is also often used in biomedical field.