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JOURNAL OF AUTOMATIC CONTROL,UNIVERSITY OF BELGRADE,VOL.19:7-12,2009
Abstract We present the design, simulation and test
results of a new AC amplifier for electrophysiologicalmeasurements based on a three op-amp instrumentation
amplifier (IA). The design target was to increase the common
mode rejection ratio (CMRR), thereby improving the quality
of the recorded physiological signals in a noisy environment.
The new amplifier actively suppresses the DC component of
the differential signal and actively reduces the common mode
signal in the first stage of the IA. These functions increase the
dynamic range of the amplifiers first stage of the differential
signal. The next step was the realization of the amplifier in a
single chip technology. The design and tests of the new AC
amplifier with a differential gain of 79.2 dB, a CMRR of 130
dB at 50 Hz, a high-pass cutoff frequency at 0.01 Hz and
common mode reduction in the first stage of the 49.8 dB are
presented in this paper.
Index Terms AC-amplifier; electrophysiological signals;CMRR; DC offset suppression
I. INTRODUCTION
HE non-invasive recordings of electrophysiological
signals are of great importance for assessing the
function of peripheral nerves, muscles, cortical activity and
the heart. The signals of interest are very small when
compared to the noise (e.g., power line interference), the
base line drift and instability, different artifacts and
electrode impedance variability [1], [2], [3] and [4]. Thestandard DC three op-amp instrumentation amplifier (IA)
circuit is one of the most popular basic configurations used
for electrophysiological amplifiers. This topology provides
high input impedance and a high common mode rejection
ratio (CMRR). High CMRR can be obtained by high gain
at the first stage of the IA and well-matched resistors in the
second stage of the IA [5]. The gain at the first stage of the
DC IA needs to be limited to prevent saturation caused by
offset potentials of electrodes.
The use of an AC amplifier instead of the DC IA
eliminates the offset. An AC amplifier based on well-
known modifications to the classic three op-amp IAtopology is achievable by various techniques [6], [7], [8],
[9] and [10]. However, the techniques suggested reduce
the performance of the DC IA.
N. Jorgovanovi is with University of Novi Sad, Faculty ofEngineering, Novi Sad, Serbia, Fax: +381 21 458873, Phone: +381 21
4852452, e-mail: [email protected]
D. Bojani is with University of Novi Sad, Faculty of Engineering,Novi Sad, Serbia Fax: +381 21 458873, Phone: +381 21 4852446, e-
mail:[email protected]
V. Ili is with University of Novi Sad, Faculty of Engineering, NoviSad, Serbia, Fax: +381 21 458873 , Phone: +381 21 4852446, e-mail:
D. Stanii is with University of Novi Sad, Faculty of Engineering,Novi Sad, Serbia, Fax: +381 21 458873, Phone: +381 21 4852452 , e-
mail:[email protected]
DOI: 10.2298/JAC0901007J
Most new electrophysiological amplifier designs use
active DC suppression. This technique relates tosubtraction of the integrated IA output (amplified DC
component of the input signal) from the original input
signal (sum of the signal of interest and undesirable DC
component). Subtraction of these two signals should not
degrade the amplifiers balanced structure or its CMRR.
The optimal solution to using this concept needs to
addresses how and when to perform the subtraction. Some
solutions for using the subtraction method are suggested
elsewhere, [11], [12], [13], [14], [15] and [16], indicating
excellent DC component suppression.
A rarely discussed factor that limits the first stage gain
of the IA and global CMRR is the dynamic range occupied
by the common mode signal. An available dynamic rangeis extremely important for battery-powered devices with
low voltage power supplies. The useful dynamic range of
the first stage could be increased by additional attenuation
of the common mode signal.
We suggest a new subtraction method for the input and
the integrated output signals. Subtraction is performed in
the first stage of the IA to preserve high input impedance
and to ensure high first stage gain and high CMRR. This
new technique reduces the common mode signal in the first
stage of the amplifier to an insignificant level, thereby
practically providing full dynamic range of the amplifiers
first stage. Moreover, common mode reduction isperformed locally in the first stage of the amplifier without
involvement of the third electrode. Therefore, the new
design can be applied with only two electrodes in the
configuration [17].
II. CIRCUIT DESIGN AND SIMULATION OF
ELECTROPHYSIOLOGICAL AMPLIFIER
A. The Novelty of the New AC AmplifierActive suppression of the common mode signal and the
DC component of the differential signal in the first stage of
the three op-amp DC IA are obtained by two feedback
loops, shown in Figure 1.The first feedback loop returns the IA output signal via
the integrator K1 and controlled voltage sources V1 and V2.
The transfer function for the differential signal is given by
( )0
1 0
( )( )
( ) 2
o DD
d D
V s A sA s
V s s K A= =
+, (1)
where AD0 is the band pass gain of the designed AC
amplifier,
2 40
1 3
21D
R RA
R R
= +
, (2)
and a lower cut-off frequency is given by
10c D
Kf A
= . (3)
An Improved AC-amplifier for Electrophysiology
Nikola Jorgovanovi, Dubravka Bojani, Vojin Ili and Darko Stanii
T
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected] -
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8 JORGOVANOVIN, ET AL.AN IMPROVED AC-AMPLIFIER FORELECTROPHYSIOLOGY
The second feedback loop returns the extracted common
mode signal from the output of the first stage of the IA VC1
via amplifier K2 and the controlled voltage source V3. If
the value of the voltage V3 is given by
13
2
12
cm
RV V
R
= +
, (4)
then the common mode voltage signal is completely
removed from the outputs of the first stage. In the
proposed circuit voltage source, V3 is controlled by the
second feedback loop (with amplifier K2). Therefore, the
expression for common mode suppression in the first stage
is
1 1 22 1
1 2 2 2
2 1
2
c
cm
V R Rfor R R
V R K R K
+= >>
+. (5)
Figure 1: Block diagram of the designed IA
As shown, the voltage sources V1 and V2 in the IA input
loop do not need to be "truly" floating. Instead of truefloating sources, it is satisfactory to provide two controlled
voltage sources referenced to the amplifiers with voltages
V3+V1 and V3-V2; this can be accomplished by using
ordinary op-amps, which will be described later.
B. Circuit Design of the AC AmplifierFor testing purposes, an AC amplifier, shown in Figure
2, was designed. The characteristics of the designed
amplifier are the following: 1) a differential gain of 79.2
dB and 2) a low cut-off frequency (0.01 Hz) and common
mode attenuation at the first stage of the IA (49.8 dB at 50
Hz). To achieve a CMRR that is as high as possible, a gain
of 59.2 dB is concentrated at the first stage. Therefore, thegain of the first stage and the gain-bandwidth product of
the op-amps define the upper cut-off frequency. The
design was based on and tested with a frequently used op-
amp (OP27) so that an upper cut-off frequency of 8.8 kHz
is achieved. The higher cut-off frequency, if required,
could be achieved with higher gain-bandwidth op-amps.
A standard three op-amp IA is based on op-amps U1, U2
and U3 and resistors R1, R2, R3, R4, R7, R8, R9, R10 and R19.
Feedback signals, linear combinations of the voltages
V3+V1 and V3-V2, are defined by two adders: U6 (R11, R13
and R15)and U7 (R12, R14 and R16). The first feedback loop
returns output voltages from the IA via two branches: 1)
attenuator R20-R21; integrator U4-R22-C1;and adder U7 (R12,
R14 and R16) (voltage V3+V1) and 2) attenuator R20-R21;
integrator U4-R22-C1; inverter U5 (R23 and R24); and adder
U6 (R11, R13 and R15) (voltage V3-V2). The second
feedback loop returns the common mode signal (Vc1) from
the output of the first stage of the IA, formed at the middle
point of the series connection of the two equal resistors R5
and R6. The Vc1 signal is buffered by op-amp U8 and
amplified by an inverting amplifier based on op-amp U9,
resistors R25 and R26 and compensation capacitor C2(voltage -V3). Gain of the inverting amplifier is set to 60
dB at DC. Due to frequency compensation, the low pass
cut-off frequency of the inverting amplifier is 15.9 Hz.
Therefore, amplification of the inverting amplifier is
reduced to 49.8 dB at 50 Hz. The output voltages of
amplifiers U6 and U7 are fed into the IA input loop via the
resistive network R17 - R18 - R19. This ensures attenuation
of the voltage difference V1-V2 by 53.5 dB and no
attenuation of voltage V3. The resistive network R17 - R18 -
R19 affects the low cutoff frequency through the constant
K1 from expression (3). The constant K1 also depends on
the integrators constant and the voltage divider R20-R21(6):
19211
20 21 22 1 17 18 19
1 RRKR R R C R R R
=+ + +
. (6)
For the values of the components shown in Figure 2, the
constant K1 is equal to 6.4910-6
, which ensures a low
cutoff frequency of 0.01 Hz. The maximum DC signal
value that can be suppressed is limited by the maximum
output voltage of the integratorVimax (defined by the power
supply voltage and op-amp output characteristics) and the
attenuation of the resistive network R17 - R18 - R19.
Additionally, Vimaxis transformed into a voltage difference
between the outputs of the two adders, attenuated through
the resistive networks and subtracted from the input
differential voltage. Maximum DC suppression can bedescribed as
19( ) max
17 18 19
in DC i
RV V
R R R=
+ +. (7)
For the components shown in Figure 2, with a power
supply of 12 V, DC component suppression is 25 mV,
which is sufficient for the recordings. The suppression can
be increased with lower attenuation in the resistive
network R17 - R18 - R19.
As we have already stated, the resistive network R17 - R18 -
R19 ensures attenuation of the differential signal, but should
have no influence on the common mode. Expression (8)
shows a relationship between the voltage on resistor R19(V19) and the common mode voltage Vc1:
11 1219 2 1
15 16
( )DS cR R
V K K V R R
= , (8)
whereK2 is the amplification of the common mode voltage
Vc1 by amplifier U9,
252
26 2 25
1
1
RK
R sC R=
+. (9)
KDS is the attenuation of the resistive network R17 - R18 -
R19:
19
17 18 19
DS
RK
R R R
=+ +
. (10)
If the ratio R11/R15 is equal to R12/R16,the voltage on R19is zero and the common mode signal will not be
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10 JORGOVANOVIN, ET AL.AN IMPROVED AC-AMPLIFIER FORELECTROPHYSIOLOGY
III. TESTING OF THE ACAMPLIFIER
To test the new topology with two feedback loops, we
built a prototype without optimizing the power
consumption, the size of the prototypes PCB area and the
circuit complexity. The reasons for skipping optimization
are the plans for integration of the amplifier into the single
chip.
Two types of prototype verification were performed: 1)verification of the amplifiers characteristics based on test
signals and 2) clinical verification by EMG recording in a
noisy environment. Data acquisition included a Tektronix
TDS3012 digital scope with a Tektronix differential
voltage probe ADA400A. A test signal was generated by
an Agilent 33220A signal generator.
A DC signal suppression test was performed by using the
sine wave input differential signal with an amplitude of 0.4
mVp-p, a frequency of 1 kHz and a DC offset of 0.8 mV.
The common mode signal on the input was set to zero in
this test. The result of the test is shown in Figure 5. The
upper waveform (Ch1) shows the differential signal at the
input of the amplifier. The bottom waveform (Ch2)represents the output of the amplifier with a sine wave
signal with an amplitude of 4 Vp-p, a frequency of 1 kHz
and a DC offset equal to zero. Test results confirm
calculations and simulations and they were similar to the
other amplifiers with active DC signal suppression.
Testing of the common mode signal reduction in the
first stage of the amplifier was performed with a high
amplitude common mode input signal. We used a sine
wave test signal with an amplitude of 15.8 Vp-p and a
frequency of 50 Hz; the differential mode signal at the
input was set to zero. The results of the CMRR test are
presented in Figure 6a. The upper waveform (Ch1) shows
the common mode signal at the input of the amplifier. The
bottom waveform (Ch2) represents the common mode
signal at the output of the first stage of the amplifier.
According to the measured results, the common mode
signal attenuation in the first stage of the amplifier is 49.8
dB at 50 Hz. This result is a significant improvement
compared to the other amplifiers that had no common
mode reduction in the first stage.
To confirm the high total CMRR of the amplifier, the
output signal of the amplifier was recorded and the results
are shown in Figure 6b. The upper waveform (Ch1) shows
the common mode signal at the input of the amplifier. The
bottom waveform (Ch2) represents the common mode
signal at the output of the amplifier. According to the
measured results, the CMRR is 130 dB at 50 Hz.
Figure 5: Verification of the differential gain and DC-offset suppression.
Ch1 shows the input signal waveform to the amplifier. Ch2 shows the
output signal waveform.
The experimental verification of the amplifier on real
signals is demonstrated by the EMG recording in an
intentionally made, extremely noisy environment. The
tested amplifier was located very close to three switch
mode power supplies and GSM phones, also it was tested
without EMI filters, shielding, or a driven right leg circuit.
We recorded EMG signals by using two Neuroline
electrodes positioned over the Flexor Digitorum m.,
following the positioning instructions defined in the
SENIAM project [18]. The neutral (i.e., ground) electrode
over the Ulnar nerve was connected directly to the
common electrode of the amplifier. The recordings from
the contracted muscle of an able-bodied individual are
presented in Figure 7a with the time scale set to 10 ms per
division. Figure 7b shows three consecutive contractions
(weak, medium and strong) from the same muscle. The
time scale (horizontal axes) is 1 second per division.
Figure 6: Verification of the common-mode rejection in the first stage of the amplifier (left). Ch1: common-mode signal at the input of the amplifier;
Ch2: common-mode signal at the output of the first stage of the amplifier; Verification of the total common-mode rejection (right). Ch1: common-
mode si nal at the in ut of the am lifier; Ch2: common-mode si nal at the out ut of the am lifier
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JOURNAL OF AUTOMATIC CONTROL,UNIVERSITY OF BELGRADE,VOL 19,2009. 11
The upper waveform (Ch1) in Figure 8 shows the same
EMG signal with a different time scale setting (i.e., 400 ms
per division). The bottom waveform represents the
frequency spectrum of the signal.
Figure 8: EMG signal of the Flexor Digitorum Muscle and its frequency
spectrum
The tests verify that the realized amplifier matches the
calculations and simulation results.
IV. CONCLUSION
The development of a new amplifiers topology that has
characteristics necessary for electrophysiological
recordings in a noisy environment was presented in this
paper. The novel topology is the active suppression of both
the DC component of the differential signal and the
common mode signal in the first stage of the three op-amp
IA. Active suppression of DC-offset was obtained by
feeding back an integrated form of the amplifier's output
that is equal to the input DC-offset, however with the
opposite sign. The active common mode signal
suppression at the first stage of the IA was obtained by
feeding back an extracted common mode signal from the
output of the first stage and presents an originalimprovement in the electrophysiological amplifiers
design.
As a result, almost the entire dynamic range of the
amplifiers first stage is available for a useful component
of the input signal. The active DC offset suppression
allows DC coupling between the electrodes and the
amplifier, providing extremely high input impedance for
the designed amplifier. The entire design of the novel AC
amplifier, with simulations and test results, were presented
in this paper. A high CMRR of 130 dB at 50 Hz, a
differential gain of 79.2 dB, a cutoff frequency at 0.01 Hz,
a bandwidth of 8.8 KHz and, in particular, a common
mode reduction of 49.8 dB in the first stage of the
amplifier were achieved. Major characteristics of the
amplifier were confirmed by the test results. With these
characteristics, this amplifier can be used to record EMG
and other electrophysiological signals. The describedcircuit is now under clinical testing as part of our virtual
EMG system [19] and [20].
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Figure 7 Short-time contraction of the Flexor Digitorum Muscle (left). Amplification of the amplifier is 79.2dB. Amplitude of the recorded signal is
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