WO2016035104A1

WO2016035104A1 - Signal acquisition system

Info

Publication number: WO2016035104A1
Application number: PCT/IT2015/000208
Authority: WO
Inventors: Marco GUERMANDI
Original assignee: Alma Mater Studiorum - Uiversita' Di Bologna
Priority date: 2014-09-04
Filing date: 2015-09-02
Publication date: 2016-03-10

Abstract

The present invention concerns an acquisition system of electrical signals (1) from a patient (P), comprising a signal acquisition unit (U1) comprising first stabilizing means (REF1) to set a first reference potential (VREF1) with respect to a first isolated ground (G1), at least two main electrodes (E3, E4), that can be applied to said patient (P), and amplification means (A3, A4, A5), having in input the signal coming from said two main electrodes (E3, E4) and, as a reference, said first isolated ground (G1), and a reference power unit (U2) comprising at least one auxiliary electrode (E2), that can be applied to said patient (P), at least one unitary gain amplifier (A2), having the input connected to said at least one auxiliary electrode (E2), said first stabilizing means (REF1) being connected in output to said at least one unitary gain amplifier (A2).

Description

Signal acquisition system

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The present invention relates to a signal acquisition system.

More specifically, the invention concerns a system for the acquisition of derived potential, for example by electroencephalographic signals (EEG), electrocardiogram (ECG), electrical impedance tomography (EIT), electromyography (EMG) and the like, with a very high common- mode rejection ratio (CMRR - Common Mode Rejection Ratio).

In the following, the description will be directed to signals in the medical field, but it is clear that the same should not be considered limited to this specific use.

As it is well known, the acquisition of electroencephalographic, electrocardiographic, tomographic, electromyographic signals, and the like requires high CMRR systems. In this context, generally the electric measurement requires a high common mode rejection ratio and the power source isolated from mains supply.

One of the main problems in the design of systems for the acquisition of bio-electrical signals is well known as common mode interference, in particular as regards the interference of signals at mains frequency of 50 or 60 Hz.

The common mode noise are generally reduced by using differential acquisition circuits, i.e., with reference to figure 1 , by means of the differential amplifier Ad, with high CMRR.

The input common mode signal of a pairs of electrodes may be converted into differential signals due to inaccuracies in the creation of the amplification stage or to impedance differences, which characterizes the contact between the electrodes and the skin.

If the common-mode rejection ratio (CMRR) of the system is not sufficiently high, the acquired signal can easily have amplitudes significantly higher than those of the useful signal.

In figure 1 , which shows a circuit according to the prior art, the potential of the common mode is reduced by attempting to force the potential on an electrode to follow the fluctuations of the potential of the isolated ground of the instrumentation by means of a low impedance connection.

The presence of a contact impedance with a finite value, as shown in the figure with Z_c, between the electrode and the patient P, however, implies that there are residual oscillations due to the effects of the resistive divider between said Z_c and the parasitic impedances to ground.

If the approach described above is not sufficient to ensure satisfactory performances, circuits referred to as Driven-Right-Leg (DRL) are used to reduce the common mode potential by acting on an electrode by means of a feedback connection (see figure 2).

The variability of the electrical characteristics of the system, on which the feedback (together with the acquisition system, of the cables, the electrodes and the patient) is carried out, requires a dimensioning of the precautionary open-chain gain of the feedback loop, in order to ensure the stability of the loop at any operating condition.

The necessity to ensure the stability of the loop limits the reduction of the common mode signal to a factor of about 10-100 times compared to the previous solution.

The solutions according to the prior art, for the always increasing sensitivity requirements of the modern electrical measurement equipment, often appear not sufficient.

Also, the systems according to the prior art require the use of components having very small manufacturing tolerances and very severe specifications, with possible necessity of calibration procedures, at the expense of the cost of the system.

Also patents n. US 4,191 ,195 and n. US 4,243,044 belong to the prior art. Patent US 4,243,044 relates to a coupling circuit with controlled ground, that removes the need for an additional electrode by driving the potential of the isolated ground and of the safety loop of the system with respect that of the patient, through a feedback based on an unitary gain amplifier. This solution presents a positive feedback loop that can give rise to instability if the circuit elements on the feedback loop are not properly controlled. In addition, also, it is required to power the unitary gain amplifier with a not isolated power supply, requiring the use of circuits for current limitation to protect the patient safety. These circuits, as known, are expensive and complicate the system design, when considering the failure risk of the components themselves.

The patent US 4,191 ,195, instead presents a solution similar to the previous one, but in that case there is a second isolated power supply for the amplifier on the feedback loop, so as to eliminate the need for current limiters. Because of the positive feedback loop, to prevent that offsets on amplifiers bring them into saturation, a capacity is inserted on the feedback loop. Two high value impedances connect isolated grounds and that of the mains network, so as to allow the discharge of any static potentials that can accumulate on the patient, causing the saturation of the amplifiers or a reduced common mode rejection. Apart from the instability risks already pointed out for the previous solution, the presence of these impedances limits the accuracy in the cancellation of the common mode potential.

In light of the above, it is, therefore, object of the present invention to propose a signals acquisition system with a common-mode rejection ratio higher than that according to the prior art systems.

It is therefore specific object of the present invention an acquisition system of electrical signals from a patient, comprising a signal acquisition unit comprising first stabilizing means to set a first reference potential with respect to a first isolated ground, at least two main electrodes, that can be applied to said patient, and amplification means, having in input the signal coming from said two main electrodes and, as a reference, said first isolated ground, and a reference power unit comprising at least one auxiliary electrode, that can be applied to said patient, at least one unitary gain amplifier, having the input connected to said at least one auxiliary electrode, said first stabilizing means being connected in output to said at least one unitary gain amplifier.

Always according to the invention, said system could comprise first supply means referring to said first isolated ground, to set a first supply potential of said acquisition units.

Still according to the invention, said first supply means could be selected from one of the following: a battery and/or provided with insulation systems through transformers and/or provided with optical insulating systems.

Further according to the invention, said reference power unit could comprise a second auxiliary electrode that can be applied to said patient, and a further amplifier having unitary gain, and an output, connected to said second auxiliary electrode, and second stabilization means to set a second reference potential in input to said additional amplifier, with respect to a second isolated ground, said second isolated ground being independent from said first isolated ground.

Advantageously according to the invention, said system could comprise second supply means referring to said second isolated ground, to establish a second supply potential for supplying said reference power unit.

Always according to the invention, said second supply means could be selected from one of the following: a battery and/or provided with insulation systems through transformers and/or provided with optical insulation systems.

Still according to the invention, said at least one unitary gain amplifier could be connected to said at least one auxiliary electrode through a capacitive coupling.

Advantageously according to the invention, said amplification means could be connected to said main electrodes through a capacitive coupling.

Further according to the invention, said differential amplification means could comprise at least one differential amplifier having an inverting input terminal and a non-inverting input terminal, each connected to one of said main electrodes, and an output terminal.

Always according to the invention, said differential signals acquisition unit could comprise at least one analog/digital converter, connected in output to said amplification means.

Still according to the invention, said amplification means could comprise a first amplifier, having an input terminal, connected to a first main electrode, and an output terminal, and at least one second amplifier having an input terminal, connected to at least one second main electrode and an output terminal.

Further according to the invention, said differential signals acquisition unit could further comprise a first analog/digital converter, connected to the output terminal of said first amplifier, and at least one second analog/digital converter, connected to the output terminal of said at least one second amplifier.

Advantageously according to the invention, said amplifiers and the respective analog/digital converters could be placed either in one or more apparatuses downstream of the cables of said main electrodes, or integrated in the main electrodes.

The present invention will be now described, for illustrative but not limitative purposes, according to its preferred embodiments, with particular reference to the figures of the enclosed drawings, wherein:

figure 1 shows a first basic scheme of differential acquisition according to the prior art;

figure 2 shows a second basic scheme of differential acquisition according to the prior art;

figure 3 shows a basic scheme of differential acquisition according to the present invention;

figure 4 shows a first embodiment of the signal acquisition system according to the present invention;

figure 5 shows a second embodiment of the signal acquisition system according to the present invention;

figure 6 shows a third embodiment of the signal acquisition system according to the present invention;

figure 7 shows a fourth embodiment of the signal acquisition system according to the present invention;

figure 8 shows an experimental configuration of the scheme of figure 4;

figure 9 shows an experimental configuration of the scheme of figure 5;

figure 10 shows a first test circuit for the connection of the electrodes to the circuits of figures 8 and 9; is

figure 11 shows a second test circuit for the connection of the electrodes to the circuits of figures 8 and 9;

figure 12 shows a further experimental configuration of the scheme of figure 5;

figure 13 shows the residue of common mode signal measured on the circuit of the experimental configuration of figure 12, when connected to the test circuit of figure 11 ;

figure 14 shows the acquisition of electroencephalographic signals with the system according to the invention with wet contact electrodes; and figure 15 shows the acquisition of electroencephalographic signals with the system according to the invention with dry electrodes.

In the various figures, similar parts will be indicated by the same reference numbers.

Referring to figure 3, it is seen a basic scheme of the signals acquisition system 1 according to the present invention.

The system 1 comprises an amplifier A1 , with unitary gain, having the own potential of the patient P in input, to which it is connected by a suitable electrode E, a contact impedance Z_c, and as output the isolated ground G1.

The system 1 also comprises a differential amplifier Ad, connected by means of electrodes (not shown in the figure) to the patient P, for which the power is related to the isolated ground G1.

In the system 1 , compared to the prior art_schemes, instead of forcing the potential of the patient P as a function of the reference potential of the isolated acquisition system 1 , the reference potential of the system 1 is fixed as a function of that of the patient P by the unitary gain amplifier A1 , powered through a second isolated power V2 and ground G2. This amplifier A1 , thus, eliminates the presence of feedback loops. If the amplifier A1 has exactly unitary gain, the common mode effects are completely eliminated, since it does not generate any voltage difference between the inputs and the reference of the differential amplifier Ad.

As it can be seen, with respect to the above mentioned patent US 4,191 ,195, in system 1 the common mode potential is detected directly on the patient, eliminating any further connection between circuits powered by a power related to the ground G and circuits powered by a power related to the ground G2. Furthermore, it is excluded the presence of feedback loops, eliminating risks of instability and the necessity of precautionary dimensioning of the components. Finally, the connection between the amplifier A1 and the ground G1 does not require a capacitive components and resistances are not inserted, which connect the isolated grounds G1 and G2 between them and the not isolated ground, which limit the operation performances.

Referring now to figure 4, it is observed a first embodiment of the signals acquisition system 1 according to the present invention, which shows mainly two portions or a first and a second unit of the circuit, indicated with the references U1 and U2, identified by the dashed lines, which are powered by means of two distinct power supplies, indicated with B1 and B2, which realize respectively a voltage V1 and V2, each relating to a respective isolated ground G1 and G2. Both these power supplies, of course, are isolated from the mains supply.

Said units U1 and U2 each comprise respectively a first stabilization device REF1 , adapted to establish a first reference potential VREF1 , and a second stabilization device REF2, adapted to establish a second reference potential VREF2, which operation will be better specified below

The isolation of said two different power supplies B1 and B2 can be obtained by any technique, which ensures compliance with the applicable safety standards, for example, to the electro-medical instrumentation. For example, battery powered systems, as in the present case, insulation systems through transformers, systems of optical isolation, etc., can be applied.

The unitary gain amplifier A1 allows to fix the potential of the patient P at a value determined by said second stabilization device REF2, which, as said above, is adapted to fix a potential VREF2 with respect to the isolated ground G2. Of course, due to the contact impedance between A1 and the patient P, as well as to the components used in order to limit the auxiliary currents, or in case of failure, the potential of the patient P at the contact point with the auxiliary electrode E1 will not be exactly equal to the second reference potential VREF2. In particular, according to the environmental conditions and the level of isolation, the amplitude of the residue interference signal at 50 Hz on the auxiliary electrode E1 can be up to about 2 Volts.

The amplifier A2, which is also unitary gain, transfers the signal on the electrode E2 (inclusive of common mode interference) and is adapted both to maximize the load impedance on the auxiliary electrode E2, as well as to minimize the impedance with which said first stabilizing device REF1 is driven, which allows to establish said first reference potential VREF1 with respect to said first isolated ground G1. Since also the power supply voltage V1 is related to the isolated ground G1 , both said voltages of the isolated ground G1 and of the power supply V1 fluctuate as the voltage VREF1 , which in its turn fluctuates as the potential of the patient P.

Therefore, the isolated ground G1 follows the potential variations on the auxiliary electrode E2, including those due to common mode interference, due for example to mains netwotk, indicated with the generator V_R. Since this link is the only low impedance connection between the units U1 and U2 of the circuit, there will not be a significant current flow between A2 and said first stabilizing device REF1 , than that necessary to balance the loss currents due to parasitic effects between the isolated grounds G1 and G2.

The set of the differential amplifier A3 and the analog/digital converter A/DC connected to its output, is an example of a system for the acquisition of the electrical signal from a pair of main electrodes E3 and E4. The output O of the analog/digital converter A/DC is connectable to the processing instrumentation of derived bio-potential signal.

Said first unit U1 of the circuit is powered by the potential difference between the voltage V1 and the isolated ground G1 . Since the latter follows the fluctuations of the potential on the auxiliary electrode E2, the common mode interference on both inputs of the differential amplifier A3 are considerably attenuated.

Figure 5 shows a second embodiment of the signal acquisition system 1 according to the present invention, in which, compared to the system 1 of figure 4, the differential amplifier A3 is replaced by a pair of amplifiers A4 and A5, powered by the power supply B 1 , and then by a difference voltage between the voltage V1 and the isolated ground G1 . Because the latter follows the fluctuations of the potential on the electrode E2, the common mode interference on the inputs of the amplifiers A4 and A5 are considerably attenuated.

To each of the amplifiers A4 and A5 a respective analog/digital converter is connected, respectively indicated with A/D-CA₄ and A/D-C-AS, to which, also in this case, the processing instrumentation of derived biopotential signals is connectable to the respective outputs O' and O".

The difference between the signals on the two electrodes E3 and E4 may be carried out downstream of the analog/digital conversion by said analog/digital converters A/D-C_A4 and A/D-CAS-

The gain differences between the amplifiers Α4 and A5 (due, for example, to the differences of electrodes and amplifiers inputs contact impedances or to manufacturing tolerances of the parameters of the components) or between the characteristics of A/D-C_A4 and A/D-C_As converters, give rise to a reduced CMRR compared to the use of a single differential amplifier (such as, for example, in the scheme of figure 4).

However, the common mode interference reduction obtained by the present invention allows to obtain levels of common mode interference sufficiently low to allow the correct acquisition of the signal.

The amplifiers A4 and A5 and the analog/digital converters A D-CA₄ and A/D-CAS can be positioned both in a single apparatus downstream of the cables of the electrodes E3 and E4, and directly on the electrodes E3 and E4 in order to achieve the so-called active electrodes.

Figure 6 shows a variation of the signals acquisition system of figure 4, in which the connection between the acquisition system 1 and the electrodes E2, E3 and E4 is not achieved by direct contact, but through a capacitive coupling C.

The auxiliary electrode E1 and the unitary gain amplifier A1 may be removed, as it is no longer necessary that the potential of the patient P is between V2 and G2, in order to avoid the saturation of the amplifier A2.

Alternatively, said auxiliary electrode E1 and the amplifier A1 can be maintained in order to reduce the fluctuations of the potential on the electrode E2 with respect to ground G2, limiting the amplitude of the input and output signals of the amplifier A2, allowing a reduction of the supply voltage V2 of the power supply B2.

Finally, figure 7 shows a further embodiment of the signals acquisition system 1 according to the present invention, which is a variant of the circuit of figure 5, in which the connection between the acquisition system 1 and the auxiliary electrode E2 and the main electrodes E3 and E4 is realized, even in this case, not by direct contact, but through a capacitive coupling.

Also in this configuration, the auxiliary electrode E1 and the unitary gain amplifier A1 may not be provided, since it is not necessary that the potential of the patient P is between V2 and G2, in order to avoid saturation of the amplifier A2. In any case, even in this case, said auxiliary electrode E1 and the respective amplifier A1 can be maintained in order to reduce the fluctuations of the potential on the electrode E2 with respect to the ground G2, limiting the amplitude of the input and output signals of the amplifier A2, allowing a reduction of the supply voltage V2.

The signals acquisition systems 1 and 2 have been subjected to tests, using a prototype based on Texas Instruments component ADS1298, which has in a single integrated circuit, the parts of signal conditioning and analog-digital conversion for 8 channels.

The two isolated power supplies are obtained by two sets of batteries B1 and B2. The acquisition of each channel is referred to a reference input. The output of the analog-digital converter is sent after appropriate circuits of electrical insulation to the computer via USB interface.

Figure 8 represents the implementation of the diagram of figure 4, where the analog-digital converter output corresponding to the first channel represents the difference between the potentials detected on the electrodes indicated by E3 and E4.

Figure 9 represents the implementation of the diagram of figure 5. The analog-digital converter outputs corresponding to the first and second channel represent the difference between the potentials detected on the electrodes, respectively indicated with E3 and E4 and a fixed potential with respect to the reference isolated ground G1. The signal difference between the two is calculated after the digital conversion of the signal.

The circuit of figure 10 is connected to the terminals/electrodes E1 , E2, E3 and E4 to perform the common mode rejection measurement. The resistors indicated with the reference R and the condensers indicated with the reference C represent a classical model of contact impedance between electrode and the patient P.

The switches S1-S4 can be operated one at a time to cancel the value of the contact impedance, so as to unbalance as much as possible the measure giving rise to the worst performance in terms of common mode rejection. The results are summarized in the table below, which lists the CMRR measured on a prototype with the test circuit of figure 10. The value between brackets indicates the CMRR value obtained at the output of the single channel CH1 , i.e. without making any difference between the signal on the electrodes E3 and E4. This represents the reduction in the common mode voltage obtained due to the present invention.

As it can be seen, in case of single-ended acquisition, most of the common mode rejection is due to the presence of the reduction circuit of the common-mode voltage (value in parentheses), which returns the CMRR value at a level substantially identical to that of the differential acquisition. In its absence, the CMRR would be less than 50 dB.

The reference standard EN 60601-2-27 requires the use of the circuit of figure 11 for the common mode rejection test. In particular, it provides that, in case of completely unbalanced circuit (any one operated between S1 , S2, S3 and S4, the others open), the output measured signal has peak to peak amplitude below 1 mV. Since the input common mode signal is equal to 10 Vrms, this specification corresponds to a CMRR of 90 dB. The following table shows the CMRR measured on a prototype with the test circuit of figure 11. Balanced load Unbalanced load

Single ended (Fig. 9) 104 dB 92,5 dB

Differenzial (Fig. 8) 105 dB 93 dB

Figure 12 shows a further experimental implementation of the acquisition system of figure 5, in which the amplification is carried out in single-ended way by means of a circuit based on operational amplifiers.

The amplification allows to relax the specifications of the analog/digital converters, which, in this case, are 16-bit instead of 24-bit as in the previous examples.

In the specific system, the amplification stage is directly integrated on the electrodes (active electrode).

Figure 13 shows the residue of common-mode signal according to the test circuit of figure 11 , which indicates a common mode rejection greater than 110 dB, over 15 dB above the limit required by the standards.

This effect is achieved despite the different gains of the amplification stages due, for example, to the input effects of the voltage divider to the amplification stage between electrode contact impedance and input of the operational amplifier, or to manufacturing tolerances of the components.

Figures 14 and 15 show the acquisition of electroencephalographic signals with the presented system. In particular, figure 15 shows the signals acquisition with wet contact electrodes, without preparation. Instead, figure 16 shows the signals acquisition with dry electrodes.

In both cases, the common mode network interference is almost completely canceled.

An advantage of the present invention is that of being directly integrated in systems for acquiring electroencephalographic signals (EEG), electrocardiogram (ECG), electric impedance tomography (EIT), electromyography (EMG) and the like, since the only modification required, with respect to common systems, is the necessity to make available a second isolated power supply. Among other things, the additional components are easily available and cheap compared to the rest of the instrumentation.

A particularly significant possible application is to use the invention to realize systems based on active electrodes, i.e. alternatively: - active electrodes constituted of an amplifier with a gain greater than 1 , to relax the specifications and reduce the acquisition cost of the system downstream;

- active electrodes containing both the amplification stage and that the analog to digital conversion.

An advantage of the system according to the present invention is that with respect to solutions based on DRL or systems according to the prior art, the attenuation of the common mode component is considerably higher, this allowing to simplify the signal acquisition circuits. In particular, it is possible to amplify the single signals before carrying out the difference among them, even without requiring very high accuracies to single-ended amplification stages, and this is particularly useful in case of systems based on active electrodes.

The present invention has been described for illustrative but not limitative purposes, according to its preferred embodiments, but it is to be understood that modifications and/or changes can be introduced by those skilled in the art without departing from the relevant scope as defined in the enclosed claims.

Claims

1 . Acquisition system of electrical signals (1 ) from a patient (P), comprising

a signal acquisition unit (U1) comprising first stabilizing means (REF1 ) to set a first reference potential (VREF1 ) with respect to a first isolated mass (G1), at least two main electrodes (E3, E4), that can be applied to said patient (P), and amplification means (A3, A4, A5), having in input the signal coming from said two main electrodes (E3, E4) and, as a reference, said first isolated ground (G1 ), and

a reference power unit (U2) comprising at least one auxiliary electrode (E2), that can be applied to said patient (P), at least one unitary gain amplifier (A2), having the input connected to said at least one auxiliary electrode (E2), said first stabilizing means (REF1 ) being connected in output to said at least one unitary gain amplifier (A2).

2. System (1 ) according to claim 1 , characterized in that it comprises first supply means (B1 ) referring to said first isolated ground (G1 ), to set a first supply potential (V1 ) of said acquisition units (U1).

3. System (1) according to claim 2, characterized in that said first supply means (B1 ) are selected from one of the following: a battery and/or provided with insulation systems through transformers and/or provided with optical insulating systems.

4. System (1) according to anyone of the preceding claims, characterized in that said reference power unit (U2) comprises

a second auxiliary electrode (E1 ) that can be applied to said patient (P), and

a further amplifier (A1 ) having unitary gain, and an output, connected to said second auxiliary electrode (E1), and

second stabilization means (REF2) to set a second reference potential (VREF2) in input to said additional amplifier (A1), with respect to a second isolated ground (G2), said second isolated ground (G2) being independent from said first isolated ground (G1 ).

5. System according to claim 4, characterized in that it comprises second supply means (B2) referring to said second isolated ground (G2), to establish a second supply potential (V2) for supplying said reference power unit (U2).

6. System (1 ) according to claim 5, characterized in that said second supply means (B2) are selected from one of the following: a battery and/or provided with insulation systems through transformers and/or provided with optical insulation systems.

7. System (1) according to anyone of the preceding claims, characterized in that said at least one unitary gain amplifier (A2) is connected to said at least one auxiliary electrode (E2) through a capacitive coupling (C).

8. System (1) according to anyone of the preceding claims, characterized in that said amplification means (A3, A4, A5) are connected to said main electrodes (E3, E4) through a capacitive coupling (C).

9. System (1) according to anyone of the preceding claims, characterized in that said differential amplification means comprise at least one differential amplifier (A3) having an inverting input terminal and a non- inverting input terminal, each connected to one of said main electrodes (E3, E4), and an output terminal.

10. System (1) according to anyone of the preceding claims, characterized in that said differential signals acquisition unit (U1) comprises at least one analog/digital converter (A/DC), connected in output to said amplification means (A3 , A4, A5).

11. System (1) according to anyone of the preceding claims, characterized in that said amplification means comprise a first amplifier

(A4), having an input terminal, connected to a first main electrode (E3), and an output terminal, and at least one second amplifier (A5) having an input terminal, connected to at least one second main electrode (E4) and an output terminal.

12. System (1) according to claim 11 , characterized in that said differential signals acquisition unit (U1) further comprises a first analog/digital converter (A/D-CA4), connected to the output terminal of said first amplifier (A4), and at least one second analog/digital converter (A/D-CA5), connected to the output terminal of said at least one second amplifier (A5).

3. System (1) according to anyone of claims 10-12, characterized in that said amplifiers (A3, A4, A5) and the respective analog/digital converters (A/D-CA4, A/D-CA5) are placed either in one or more apparatuses downstream of the cables of said main electrodes (E3, E4), or integrated in the main electrodes (E3, E4).