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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:

[email protected]

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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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

2.2mVp-p; EMG signal of Flexor Digitorum Muscle (right). Amplification of the amplifier is 79.2dB. Amplitude of the low contraction is 0.6mVp-p,

amplitude of the medium contraction is 1mVp-p and amplitude of the high contraction is 2mVp-p

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