Device for detecting electrical potentials

Abstract

Device for detecting electrical potentials with a plurality of measuring inputs (7) for connection to measuring electrodes (9) which can be placed on the body of a patient (3), with a plurality of measuring amplifiers (Op1,...,Op N ), which have a first and a second input and an output (11), and with a summing unit (13, 23) connected to the outputs (11) of the measuring amplifiers (Op1,...,Op N ) is connected and is designed to produce a signal proportional to the mean of the signals of the outputs (11) of the measuring amplifiers (Op1,...,Op N ) to output at an output (15, 17) of the summing unit (13, 23), with a first amplifier (Op c ), whose input is connected to the output (15) of the summing unit (13, 23), and wherein each of the measuring inputs (7) is connected to a first input of a measuring amplifier (Op1,...,Op N ) is connected, where the second input of each measuring amplifier (Op1,...,Op N ) is connected to the output (17) of the summing unit (13, 23), characterized in that that the device (1, 1') has a potential output (19) for connection to an electrode (21) which can be placed on the body of the patient (3) and that the output of the first amplifier (Op c ) is connected to the potential output (19), where the combination of the summing unit (13, 23) and the first amplifier (Op c ) provides an inverted, amplified mean value signal at the potential output (19).

Description

[0001] The present invention relates to a device for detecting electrical potentials, for example, of a patient's body, comprising a plurality of measuring inputs for connection to measuring electrodes that can be placed on the patient's body, a plurality of measuring amplifiers having a first and a second input as well as an output, and a summing unit connected to the outputs of the measuring amplifiers and configured to output a signal proportional to the average of the signals from the outputs of the measuring amplifiers at an output of the summing unit, wherein each of the measuring inputs is connected to a first input of a measuring amplifier and wherein the second input of each measuring amplifier is connected to the output of the summing unit. The present invention further relates to a method for detecting electrical potentials.

[0002] For example, if electrical potentials are to be measured on a patient's skin and the useful signal contained in these potentials is only in the µV range, as can be the case with an electrocardiogram (ECG), an electromyogram (EMG), electroencephalography (EEG) or electrooculography (EOG), the following problems arise.

[0003] Since the patient's body is surrounded by electric fields, potentials form on the patient's skin solely due to capacitive coupling. This effect can be generally described as the body being capacitively coupled, in particular, to a 230V / 50Hz alternating current field caused by mains voltage sources in the patient's vicinity. However, for safety reasons, it is not permissible to couple the patient directly to a uniform environmental ground, as this would pose a significant risk to the patient.

[0004] Furthermore, for safety reasons, the measuring device to which the electrodes on the patient's skin are connected must also be galvanically isolated from the surrounding ground. This, in turn, means that the measuring device, with its internal ground, is also capacitively coupled to the environment, creating the problem that the device ground is at a potential of unknown magnitude and generally different from the patient's potential.

[0005] In order to at least ensure that the patient and the mass of the measuring device are at the same potential, or at least that there is a fixed potential difference between them, it is known to connect the device mass and the patient's body via an additional electrode.

[0006] However, since the device ground and the patient can generally be at different potentials due to the inhomogeneity of the surrounding fields, resulting from the different capacitive coupling to the environment, a compensating current flows. Due to the impedance of the coupling to the patient, this current leads to a so-called common-mode signal via the electrode, which is then amplified by the amplifiers in the measuring device. If the actual signal to be measured is very small, the common-mode signal prevents the actual signal from being resolved. Furthermore, the amplifiers must have a high input dynamic range to process both the signal and the superimposed, higher-order common-mode signal. Additionally, downstream digital evaluation electronics must provide a high number of bits per measurement to process these large signals.

[0007] It is known from Bruce B. Winter et al., Driven-Right-Leg Circuit Design, IEEE Transactions on Biomedical Engineering, Vol. BME-30, No. 1, January 1983, that the additional electrode attached to the patient can be supplied with a potential by the measuring device that corresponds to the mean of the signals acquired at the measuring electrodes, with this mean signal being further amplified. To significantly suppress the common-mode signal in this way, high gains for the mean signal are required, which is difficult to achieve. In particular, the problem arises that high gains lead to oscillations in the output signal.

[0008] Furthermore, it is also known from DE 29 26 165 A1, from which the present invention is based, to subtract the average value of the signals output by the amplifiers from the input signals of the amplifiers. However, this presents the problem that the common-mode signal, although not amplified, is nevertheless output together with the useful signal at the amplifier's output. If the useful signal is extremely small, this can lead to the amplitude of the common-mode signal and that of the amplified useful signal being of the same order of magnitude, so that they cannot be easily separated. In addition, there is the problem that the amplifiers and any downstream evaluation electronics must be adapted to further process the comparatively large common-mode signal.

[0009] Based on this, the object of the present invention is to design a measuring device for detecting potentials in such a way that common-mode signals are reliably removed from the signal.

[0010] This task is solved by providing the device with a potential output for connection to an electrode that can be placed on the patient's body, and by providing a first amplifier whose input is connected to the output of the summing unit and whose output is connected to the potential output.

[0011] In the device according to the invention, the average value signal output by the summing unit is fed back to the second input of each measuring amplifier and subtracted there from the directly acquired input signal, thereby achieving a first reduction of the common-mode signal component in the output signal at the measuring amplifiers. Secondly, the average value signal is fed to a first amplifier, the output of which is connected to the potential output connected to an additional electrode on the patient. This amplifies the average value signal, which also has the effect of further reducing the common-mode signal in the output signal at the outputs of the measuring amplifiers.

[0012] By combining these two principles, a significant reduction in the common-mode signal component can be achieved without the need for high gain in the branch between the summing unit and the potential output to which the additional electrode is connected.

[0013] Even when the average signal is applied unamplified to the auxiliary electrode, the common-mode signal is reduced or suppressed to twice the extent compared to operating without the first amplifier and auxiliary electrode. A moderate gain of 10 for the first amplifier further reduces the common-mode signal suppression by a factor of 11 compared to omitting the first amplifier. Thus, even small gains in the branch to the auxiliary electrode can achieve a significant effect. Furthermore, this approach ensures that the overall common-mode signal is smaller and that the input voltage range of the measuring amplifiers is not exceeded. This can easily occur if the average signal is fed back only to the inputs of the measuring amplifiers.This effect results, as will be described in detail in connection with the preferred embodiments, from the interaction of the feedback of the mean value signal to the second inputs of the measuring amplifiers and the supply of the mean value signal in amplified form to the additional electrode.

[0014] In a preferred embodiment, the summing unit is designed as a microprocessor unit which is programmed to output a signal at a first output, the magnitude of which corresponds to the average of the signals of the outputs of the measuring amplifiers, wherein the input of the first amplifier is connected to this first output of the microprocessor unit. It is further preferred if the microprocessor unit has a second output, which also outputs a signal whose amplitude corresponds to the average of the signals from the outputs of the measuring amplifiers, with this signal being amplified by a factor V. The second inputs of the measuring amplifiers are connected to this second output of the microprocessor unit, so that the common-mode signal suppression can be further adjusted by selecting the gain V.

[0015] Finally, the summing unit can also be implemented using analog technology by incorporating a second amplifier. One input of this amplifier is connected to the outputs of the measuring amplifiers in such a way that the output of the second amplifier produces a signal whose amplitude corresponds to the average of the signals from the outputs of the measuring amplifiers, i.e., is equal to or at least proportional to the average. It is clear to those skilled in the art how such an averaging circuit can be implemented analogously using an amplifier or operational amplifier. Furthermore, the output of this second amplifier is then connected to the second inputs of the measuring amplifiers and to the input of the first amplifier, the latter of which can then be connected to the additional electrode on the patient.

[0016] According to a second aspect of the present invention, the aforementioned problem is solved by a method for detecting electrical potentials, wherein the potentials are detected as input signals, each fed to a first input of a plurality of measuring amplifiers, and amplified to output signals, an average value signal of the output signals being generated, the amplitude of which is proportional to the average value of the output signals, the average value signal being fed to second inputs of the measuring amplifiers, and the average value signal being supplied to a potential output connected to an additional electrode. This method combines the advantages already described in connection with the device according to the invention.

[0017] The present invention will now be explained with reference to a drawing showing only preferred embodiments, wherein Fig. 1 a schematic representation of a first embodiment of a device according to the invention for detecting potentials and Fig. 2 shows a schematic representation of a second embodiment.

[0018] In the illustration of the first embodiment of a device 1 according to the invention for detecting potentials, in this case potentials on the skin of a patient 3 (shown only schematically), the capacitances C1, C2, C3, and C4 initially indicate that both the patient 3 and the housing 5, which forms the ground m2 of the device 1, are capacitively coupled to the environment and, in particular, to a 50 Hz alternating voltage field. This field results in the patient 3, on the one hand, and the housing 5, or the device ground m2 of the device 1 connected to it, on the other hand, being able to have different potentials.

[0019] The device 1 has a plurality of measuring inputs 7, to which input signals V1,...,V are connected. N can be recorded, whereby the measuring inputs 7 can be connected to electrodes 9 on the skin of the patient 3 and the connection between the measuring inputs 7 and the skin of the patient 3 has an impedance Z1,...,Z N has.

[0020] The device 1 further features a number of measuring amplifiers Op1,...,Op corresponding to the number of measuring inputs 7. N on, whose first, non-inverting input is each connected to a measurement input 7. The outputs 11 of the measurement amplifiers Op1,...,Op N They serve, on the one hand, to compare the respective input signal V to the input signal V i amplified output signal E ito tap off a measure of the temporal evolution of the potential on the skin of patient 3. On the other hand, the outputs 11 are connected to a summing unit, which in this first preferred embodiment is designed as a microprocessor unit 13.

[0021] The microprocessor unit 13 is programmed in such a way that it can derive from the output signals E1,...,E output at the outputs 11 N a mean value signal is generated, the level of which corresponds to the mean value of the signals E output at outputs 11. i This means that the average value signal is output on the one hand by the microprocessor unit 13 at a first output 15, and on the other hand the average value signal, possibly amplified by a factor V=β / (α-1), is output at a second output 17.

[0022] The first output 15 of the microprocessor unit 13 is connected to the inverting input of a first amplifier op. cconnected, whose non-inverting input is connected to the device ground m2 of the facility. 1 The first amplifier Op c It is thus connected as an inverting amplifier, and the gain γ can be adjusted via resistors R1 and R2, with which the average signal is used as an amplified reference signal V. c at a potential output 19 of the device 1, with which the output of the first amplifier Op c is connected, is output. Here it is also conceivable that the first amplifier op c It is configured as a non-inverting amplifier. In that case, the signal supplied to it would have to be inverted beforehand by the microprocessor unit 13. The potential output 19 is connected to an additional electrode 21 on the patient 3.

[0023] The second output 17 of the microprocessor unit 13 is connected via the resistors R to the second, inverting input of the measuring amplifiers Op1,...,Op nconnected, so that the possibly amplified average signal is derived from the input signal V1,...,V N , which is captured by the measurement inputs 7, is subtracted.

[0024] This first preferred embodiment works as follows.

[0025] Since the measuring amplifiers Op1,...,Op N When connected as non-inverting amplifiers, the input signal V1,...,V detected at the measuring inputs 7 is N with a factor α=1+R'R ver− amplified at outputs 11 of the measuring amplifiers Op1,...,Op N issued.

[0026] However, the mean signal is subtracted from this. Egg, However, this signal is amplified by a factor α-1. Therefore, the output signal E1,...,E N at output 11 of the measuring amplifiers Op1,...,Op N : Ei=α⋅Vi−βα−1⋅(α−1)⋅Ei¯

[0027] If the mean Egg Ei for the output signals E i The following relationship results from the formation of the data: Ei¯=α⋅Vi¯−β⋅Ei¯

[0028] Since the agent Vi¯ the common-mode signal V cm This corresponds to the result Ei¯=α1+β⋅Vcm.

[0029] Since the signal at the measuring inputs 7 is derived from the actual signal Vi¯ and the common-mode component V cm When composed of different elements, the first relationship can also be described as... Ei=α⋅(V^l+Vcm)−β⋅α1+β⋅Vcm or Write Ei=α⋅V^l+α1+β⋅Vcm.

[0030] The last relationship implies that the difference between two input signals V i at outputs 11 of the measuring amplifiers Op1,...,Op N by a factor |A diff |= α is amplified, while the common-mode signal is amplified by the factor |A gleich | = α / (1+β) is amplified.

[0031] For common-mode rejection CMRR = |A diff | / |A gleich This results in CMRR = (1+β). Due to the difference calculation and inaccuracies in the resistances R and R', another factor, CMRR, is added. diff in addition, so that overall the relationship CMRR applies to common-mode rejection. ges = (1+β) · CMRR diff results.

[0032] These considerations apply without taking into account that the mean value signal amplified by the gain y is transmitted via the additional electrode 21. Egg this is applied to the patient. This leads to an overall damping of the common-mode component, so that then for the common-mode component V cm,ges , which is then to be used in the previous equations, the following applies: Vcm,ges=Vcm⋅11+G with G=γ⋅α1+β.

[0033] Then the total common-mode rejection ratio (CMRR) is obtained. ges the relationship CMRR'tot=CMRRtot⋅(1+G)=CMRRdiff⋅(1+β)⋅(1+γ⋅α1+β)=CMRRdiff⋅(1+β+γα).

[0034] If the gains at the measuring amplifiers Op1,...,Op N as well as for the second, inverting inputs of the measuring amplifiers Op1,...,Op N If the incoming mean signal is chosen such that β = α-1, this relationship simplifies further to CMRR'tot=CMRRdiff⋅α⋅(1+γ).

[0035] This means that even when the mean signal is supplied to the auxiliary electrode 21 without amplification, i.e., γ = 1, the common-mode rejection ratio is doubled. Amplifying the mean signal by a factor of 10, i.e., γ = 10, leads to an increase in the common-mode rejection ratio by a factor of 11.

[0036] This shows that, through the inventive design or the combination of the feedback of the average value signal to the second inverting input of the measuring amplifiers Op1,...,Op Nas well as the feeding of the mean value signal to an additional electrode 21, the common-mode signal can be strongly suppressed without requiring high amplification: This results precisely from a combination effect of the two feedbacks, and does not occur independently of each other, as the previous considerations show.

[0037] The in Fig. The second embodiment of a device 1' according to the invention for detecting potentials, as shown in Figure 2, differs from the first embodiment only in that the summing unit is not implemented as a microprocessor unit, but has a second amplifier 21, which is configured as a non-inverting amplifier and at whose output an average value signal is provided, the magnitude of which corresponds to the average value of the signals at the outputs 11 of the measuring amplifiers Op1,...,Op N output signals E1,...,E NThis corresponds to the average value signal, which, as in the first embodiment, is amplified if necessary and fed to the second inverting inputs of the measuring amplifiers Op1,...,Op N fed to it. On the other hand, this average signal is in turn fed to the first amplifier Op. c supplied and is amplified to) the potential output 19 and supplied to the additional electrode 21.

[0038] Thus, in this setup as well, 7 potentials are measured at the measuring electrodes as input signals V1,...,V N captured, the measuring amplifiers Op1,...,Op N fed in, where they are amplified to output signals E1,...,E N will be, and an average signal of these output signals will be generated) E1,...,E N This average signal is then generated. This average signal is then fed back to the inputs of the measuring amplifiers Op1,...,Op N as well as the first amplifier op cThe signal is routed. This achieves the same advantages already explained in connection with the first embodiment. Here, only the summing unit is implemented analogously, using an amplifier 23, and not digitally by means of a microprocessor unit 9. Reference symbol list: 1, 1' facility 3 patients 5 cases 7 Measuring input 9 electrode 11 Output - Measuring Amplifier 13 Microprocessor unit 15 First output - Microprocessor unit 17 Second output - Microprocessor unit 19 Potential output 21 Additional electrode 23 second amplifier V i Input signal E i Output signal Op i Measuring amplifier Op c first amplifier

Claims

[1] Device for detecting electrical potentials with a plurality of measuring inputs (7) for connection to measuring electrodes (9) which can be placed on the body of a patient (3), with a plurality of measuring amplifiers (Op1,...,Op N ), which have a first and a second input and an output (11), and with a summing unit (13, 23) connected to the outputs (11) of the measuring amplifiers (Op1,...,Op N ) is connected and is designed to produce a signal proportional to the mean of the signals of the outputs (11) of the measuring amplifiers (Op1,...,Op N ) to output at an output (15, 17) of the summing unit (13, 23), with a first amplifier (Op c ), whose input is connected to the output (15) of the summing unit (13, 23), and wherein each of the measuring inputs (7) is connected to a first input of a measuring amplifier (Op1,...,Op N ) is connected, where the second input of each measuring amplifier (Op1,...,Op N ) is connected to the output (17) of the summing unit (13, 23), characterized by , that the device (1, 1') has a potential output (19) for connection to an electrode (21) which can be placed on the body of the patient (3) and that the output of the first amplifier (Op c ) is connected to the potential output (19), where the combination of the summing unit (13, 23) and the first amplifier (Op c ) provides an inverted, amplified mean value signal at the potential output (19). [2] Device according to claim 1, wherein the summing unit is designed as a microprocessor unit (13) which is programmed to output a signal at a first output (15) of the microprocessor unit (13), the magnitude of which corresponds to the average of the signals (E1,...,E N ) the outputs (11) of the measuring amplifiers (Op1,...,OpN ) corresponds, and where the input of the first amplifier (Op c ) is connected to the first output (15) of the microprocessor unit (13). [3] Device according to claim 2, wherein the microprocessor unit (13) has a second output (17) at which a signal is output, the magnitude of which is the average of the signals (E1,...,E N ) the outputs (11) of the measuring amplifiers (Op1,...,Op N ) increased by a factor V corresponds to, and wherein the second output (17) of the microprocessor unit (13) is connected to the second inputs of the measuring amplifiers (Op1,...,Op N ) is connected. [4] Device according to claim 1, wherein the summing unit has a second amplifier (23) whose input is connected to the outputs (11) of the measuring amplifiers (Op1,...,Op N ) is connected in such a way that a signal is output at the output of the second amplifier (23) whose amplitude corresponds to the average of the signals (E1,...,EN ) the outputs (11) of the measuring amplifiers (Op1,...,Op N ) corresponds, and where the output of the second amplifier (23) is connected to the second inputs of the measuring amplifiers (Op1,...,Op N ) and with the input of the first amplifier (Op c ) is connected. [5] Methods for measuring electrical potentials, where the potentials are used as input signals (V1,...,V N ) detected, each a first input of a plurality of measuring amplifiers (Op1,...,Op N ) fed to and to output signals (E1,...,E N ) be reinforced, where an average signal of the output signals (E1,...,E N ) is formed, the height of which is proportional to the mean of the output signals (E1,...,E N ) is, and where the average signal is taken from the second inputs of the measuring amplifiers (Op1,...,Op N ) is forwarded, characterized by , that the mean value signal is inverted and amplified and fed to a potential output (19) which is connected to an additional electrode (m1).

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