Paper No. T5-1.4, pp. 1-5
PSU-UNS International Conference on Engineering and
Environment - ICEE-2005, Novi Sad 19-21 May, 2005
University of Novi Sad, Faculty of Technical Sciences
Trg D. Obradovi)a 6, 21000 Novi Sad, Serbia & Montenegro
Design and improvement of AC-Coupling
Instrumentation Amplifier for Biopotential
Measurements
Noi SOMALA§, Sawit TANTHANUCH, Booncharoen WONGKITTISUKSA
and Chusak LIMSKUL
Department of Electrical Engineering, Faculty of Engineering, Prince of Songkla University,
Hatyai, Songkhla, Thailand, 90112
E-mail: s4512032@psu.ac.th §
electrode grounded amplifiers needed high CMRR and
common-mode input impedance.
Pallas-Areny et al. [3] proposed a solution to provide
high input impedances, even in the presence of
grounding resistors. It consisted of an ac bootstrapped
buffer that provided high input impedance and ac
coupling. However, this arrangement was not well suited
for low noise applications. There was unity gain and
contributes with noise.
Spinelli and Mayosky[4] proposed the use of
optocouplers in photovoltaic mode and an integrator,
included in a negative feedback loop, for input DC
voltage compensation and high-pass filtering. The
optocoupler transfer characteristics were non-linear, and
there was a wide variation between specimens of the
current-to-current transfer ratio (about twice). This
leaded to low accuracy of the high-pass cutoff frequency.
In this paper, we proposed a novel biopotential
amplifier enhanced to reduce half-cell potential and
common-mode signal. The proposed circuit was
implemented and verified with OP27AZ and
TLC2652CP.
Abstract: AC coupling is essential in biopotential
measurements. Electrode offset potentials can be several
orders of magnitude larger than the amplitudes of the
biological signals, thus limiting the admissible gain of a
dc-coupled front end to prevent amplifier saturation.
This paper presents the improvement of biopotential
amplifier to suppress half-cell potential and increase
common-mode rejection ratio (CMRR). The proposed
circuit used 6 op-amps, 4 op-amps for preamplifer,
1 op-amp for differential amplifier and 1 op-amp for
integrator amplifier, Results show that the design meet
± 66 mV for half-cell potential rejection, 46 dB for
differential gain and 154.32 dB for CMRR. The design
will be intended to use in various applications, such as
Holter-type monitors, defibrillators, Electrocardiogram
(ECG) monitors, biotelemetry devices etc.
Key Words: Biopotential amplifier /Half-cell potential
/ High CMRR
1. INTRODUCTION
The three op-amps instrumentation amplifier (IA),
differential input and single-ended output, is one of the
most versatile front-end biopotential amplifier because it
presents high input impedances and high gain. The
equivalent input noise in this condition depends
exclusively on the two op-amps comprising the input
stage. High gain at the first stage is a condition difficult
to achieve in bioelectric measurements, mainly due to
electrode offset voltages and interference.
Winter and Webster [1] considered interference
reduction in both isolated and nonisolated amplifiers.
They proposed to reduce interference by increasing the
amplifier’s effective CMRR or by reducing the commonmode gain by increasing the isolation impedance.
Thakor and Webster [2] analyzed power line
interference in two-electrode ECG recordings. Groundfree amplifiers were safer than grounded amplifiers, and
two-electrode amplifiers were common in biotelemetry
and ambulatory monitoring. They realized that in twoelectrode amplifiers the interference is larger in grounded
amplifiers than in ground-free amplifiers. Hence, two-
2. PRINCIPLE OF INSTRUMENTATION
AMPLIFIER
2.1 Differential amplifiers[5]
Fig. 1 shows a simple one op-amp differential
amplifier. Current flows from v2 through R1 and R2 to the
ground and no current flows into the op amp. Thus, R1
and R2 act as a voltage divider.
The voltage that appears at the positive input terminal
determines the voltage at the negative terminal. The top
part of the amplifier works like an inverter.
The voltage at the positive input terminal is
v3 =
1
R2
v2 .
R1 + R2
(1)
R1
R2
2.2 Conventional instrumentation amplifier
Fig. 2 shows conventional instrumentation amplifier
constructed from a buffered differential amplifier stage
with three new resistors linking the two buffer circuits
together [5]. It is beneficial to be able to adjust the gain
of the amplifier circuit without having to change more
than one resistor value, as is necessary with the previous
design of differential amplifier.
v1
R1
v3
A
v3
v2
+
vo
R2
Fig. 1. A differential amplifier uses two active inputs
and a common connection.
Since there is no flow of current into the op amp,
v v
v v
i= 1 3 = 3 o .
R1
R2
(2)
Combining these two equations yields,
R
v o = 2 (v 2
R1
Fig. 2. Conventional instrumentation amplifier (IA.)
v1 ) .
(3)
This is the equation for a differential amplifier. If the
two inputs are hooked together and driven by a common
source, then the common-mode voltage is equal to Vc (v1
= v2). The differential amplifier circuit gives an output of
0 and the differential amplifier common-mode gain (Ac)
is 0. When v1 B v2, the differential voltage (v2 – v1)
produces a differential gain (Ad), which equals R2/R1 .
In some instances, the signal on both input lines is
corrupted by a surrounding common mode interference
and is not acceptable for us to process. Eq. (3) indicates
that different input signals of the differential amplifier
are amplifier and the common interference on both
inputs will not be amplified.
However, no differential amplifier perfectly rejects
the common-mode voltage. To quantify the performance
of a differential amplifier, we use the term commonmode rejection ratio (CMRR), which is defined as
CMRR
=
Ad
.
Ac
(4)
In measurement applications, a CMRR greater than
100 may be acceptable but a high-quality differential
amplifier may have a CMRR greater than 10,000 or 80
dB.
To measure CMRR, we have to know both Ad and
Ac. We can apply the same voltage to both op-amps at
inputs and measure the output to obtain Ac. Then we can
connect one input to ground and apply a voltage to the
other one. From the output, we can obtain Ad. With these
two numbers, we can calculate the CMRR from Eq. (4).
As with the inverting amplifier, we must be careful
that external resistances of the op-amp circuit do not
affect the circuit.
Consider all resistors to be of equal value except for
Rg. The negative feedback of the upper-left op-amp
causes the voltage at point 1 (top of Rg) to be equal to
V1. Likewise, the voltage at point 2 (bottom of Rg) is
held to a value equal to V2. This establishes a voltage
drop across Rg equal to the voltage difference between
V1 and V2. The voltage drop causes a current through Rg,
and since the feedback loops of the two input op-amps
draw no current, that same amount of current through Rg
must be going through the two "R1" resistors above and
below it. This produces a voltage drop between points 3
and 4 :
V3-4
=
(V2 - V1 ) 1 + 2 R1
Rg
.
(5)
The regular differential amplifier on the right-hand
side of the circuit then takes this voltage drop between
points 3 and 4, and amplifies by a unity gain. It has the
distinct advantages of possessing extremely high input
impedances on the V1 and V2 inputs (because they
connect straight into the non-inverting inputs of their
respective op-amps), and adjustable gain that can be set
by a single resistor. Manipulating Eq. (5), we have a
general expression for overall voltage gain of the IA:
Ad
=
1+
2 R1
.
Rg
(6)
3. PROPOSED CIRCUIT
3.1 CMRR enhancement[6]
The CMRR enhancement circuit is shown in Fig 3.
The input stage consists of operational amplifiers A1,
A2, A3 and A4. A1 and A2 are the main gain stages, A3
and A4 are unity gain buffers. As the non-inverting input
voltages of A3 and A4 are equal to their respective
output voltages. A5 at the output is a 4-input unity gain
differential amplifier. The output voltage (Vout ) and
differential gain (Ad) are following :
Vout = Ad (Vin + Vin
Ad
=
).
2 R2 R3 + 2 R1R2 + 2 R1R3
.
R3 Rg + 2 R1R3
(7)
(8)
offset voltage and high CMRR types; A1 and A2 to be of
high open-loop gain, high CMRR and high gainbandwidth type.
3.2 DC suppression enhancement[7]
The principle of the ac-coupled instrument amplifier
is to maintain the mean output voltage at zero. The
circuit shown in Fig. 4(a) provides an integrator
amplifier circuit and the modeling is shown in Fig. 4(b).
Through, the transfer function , is given by
=
Hence R1 » R2 » Rg ,
Ad
= 1+
R2
.
R3
Vout
Vin
=
1
sCR5
(11)
(9)
(a)
(b)
Fig. 4. (a)Integrator amplifier circuit. (b) A
modeling of integrator amplifier circuit.
Fig. 3. CMRR enhancement circuit
The first stage has unity common mode voltage gain.
The second stage has unity differential mode voltage
gain. The minimum CMRR can be calculated as [6]
CMRR =
=
According to Fig. 3, the first stage gain and the
second stage gain, unity gain, are consequently denoted
, is
with Ad and 1. The integrator amplifier circuit
applied to CMRR enhancement circuit for negative
feedback loop shown in Fig. 5(a) and the open loop gain
is simplified as shown in Fig. 5(b).
Ad 1 4 Ad 5
Acm1 4 Acm5
Ad
1
1 4 / (1 + R4 / 2 R4 )
= 0.375
(a)
(10)
Ad
(b)
where :
/
= the tolerance of the R4 resistors used.
= differential gain
Ad
Acm = common mode gain
Fig. 5. (a) Negative feedback configuration.
(b) Open loop gain simplified
The
If Ad = 1000 and / = 1%, the theoretically computed
minimum CMRR (assuming ideal operational amplifiers)
is 91.5 dB, taking opposite signs for the resistor
tolerances. With Ad = 200, CMRR becomes 77.5 dB.
Taking into on account for realistic op-amps (CMRR min
is 75 dB), Ad = 200 and / = 5%, CMRR become
63.5 dB.
When selecting operational amplifiers, the following
should be respected: A3, A4 and A5 to be low input
can be calculated as
=
1
1+
.
(12)
Therefore, the overall gain corresponds to a first
order high-pass filter with a single pole in the results:
sCR5
.
1 + sCR5
(13)
f c , 3 dB =
1
.
2 CR5
(14)
50
45
40
35
30
25
20
15
10
5
0
0.01
Output Voltage (dB)
Ad
G =
Finally, the proposed circuit can be achieved with the
arrangement shown in Fig. 6.
IA
0.1
1
Proposed
10
100
1000
10000
100000
Frequency (Hz)
Fig. 8. Differential gain of conventional
instrumentation amplifier and the proposed circuit as a
function of frequency
IA
Output Voltage (dB)
0
-20
-40
-60
-80
-100
-120
-140
0.01
0.1
1
4. EXPERIMENTAL RESULTS
The proposed circuit was implemented as shown in
Fig. 7. We used OP27AZ for A1, A2, A5 (1.8 million
loop gain and 126 dB CMRR) and TLC2652CP for A3,
A4, A6 (1 µv DC offset voltage and 120 dB CMRR).
All Resistors are 5% tolerance (R1 = 1 ML,
Rg = 200 L, R2 = 200 kL, R3 = 1 kL, R4 = 100
kL and R5 = 1.6 ML ). Capacitor is 1MF. In the
results, the design meet 46.06 dB for AC gain, -108.26
dB for common mode gain and 154.32 dB for CMRR as
shown in Table. 1 and Fig. 8 –12.
10
100
1000
10000 100000
Frequency (Hz)
Proposed circuit
Fig. 9. Common-mode gain of conventional
instrumentation amplifier and the proposed circuit as a
function of frequency
CMRR (dB)
Fig. 6.
Proposed
IA
180
160
140
120
100
80
60
40
20
0
0.01
0.1
1
10
Proposed
100
1000
10000 100000
Frequency (Hz)
Fig. 10. CMRR of conventional instrumentation
amplifier and the proposed circuit as a function of
frequency
Input Voltage
Output Voltage
Fig. 7. Implemented circuit
Table 1. Comparison of Differential gain in Eq. (8)
and implemented circuit
Condition
Theoretical analysis (eq.8)
Implemented circuit
Gain(dB)
46.072
46.060
Fig. 11. A plot of input signal(Vd=1mV,
100 Hz+Vcm=10mV,50 Hz) and output signal
Input Voltage
Output Voltage
Fig. 12. A plot of input signal (Vd=1mV,100 Hz +
Vcm=10mV,500 Hz) and output signal
5. CONCLUSIONS
We have shown that 6 op-amps IA can achieve
higher performance than conventional instrumentation
amplifier. The overall gain is ensured by the first stage;
thus a high CMRR is obtained with no need of highprecision resistors in the second stage and half-cell
potential is reduced with feedback integrator circuit. The
implemented circuit is found 154.32 dB for CMRR and
± 66 mV for DC suppression with 34 kHz bandwidth.
This makes it suitable used for precision medical
measurement systems.
6. REFERENCES
[1] B. B. Winter and J. G. Webster, “Reduction of
interference due to common mode voltage in
biopotential amplifiers,” IEEE Trans. Biomed.Eng.,
vol. BME-30, pp. 58–62, 1983.
[2] N. V. Thakor and J. G. Webster, “Ground-free ECG
recording
with two electrodes,” IEEE Trans.
Biomed. Eng., vol. BME-27, pp. 699–704, 1980.
[3] R. Pallás-Areny, J. Colominas, and J. Rosell, “An
improved buffer for biolectric signals,” IEEE Trans.
Biomed. Eng., vol. 36, pp. 490–493, Apr. 1989.
[4] Enrique M. Spinelli and Miguel Angel Mayosky,
”AC Coupled
Three
op-amp
Biopotential
Amplifier with Active DC Suppression”, IEEE
Trans. Biomed. Eng., vol.47, pp. 1616-1619,2000.
[5] Webster, J. G. (ed.) 1998. Medical Instrumentation:
Application and Design. 3rd ed. New York: John
Wiley & Sons.
[6] Dobromir Dobrev,
“Two-electrode low supply
voltage electrocardiogram signal”, Med. Bio. Eng.
Comp., vol. 42, pp. 272–275, 2004.
[7] Stitt, Mark, “AC-Coupled Instrumentation and
Difference Amplifier,” Burr-Brown, AB-008,May
1990.