Device for detecting electric potentials

Abstract

A device for detecting electric potentials includes a plurality of measuring inputs (9) for connecting to measuring electrodes (11), which can be placed on the body of a patient (3), a plurality of measuring amplifiers (Op.sub.1, . . . , Op.sub.N), and a potential output (27) for connecting to an additional electrode (31), which can be placed on the body of the patient (3), to which a preset voltage can be applied. A summing unit (17) sends a signal, which is an indicator of the mean value of the signals sent by the measuring amplifiers (Op1, . . . , OpN). A current-measuring device (29) sends a current signal, which is proportional to the current flowing through the potential output. An analyzing unit (35) is connected to receive a potential output voltage signal, the summing unit output (19) signal and the current-measuring device signal. The analyzing unit is configured to generate an impedance signal from the fed signals.

Claims

1. A device for detecting electric potentials, the device comprising: a plurality of measuring inputs for connecting to measuring electrodes, the measuring electrodes being adapted to be placed on the body of a patient; a plurality of measuring amplifiers, each of the plurality of measuring amplifiers being associated with a respective one of the measuring inputs and each of the measuring inputs being connected to an input of the associated measuring amplifier; a potential output for connecting to an additional electrode, which can be placed on the body of the patient, the potential output being designed such that a preset voltage can be applied thereto; a summing unit connected to the outputs of the measuring amplifiers, the summing unit being configured to send a signal, which is an indicator of the mean value of the signals sent by the measuring amplifiers; a current-measuring device connected to the potential output, the current-measuring device being configured to send a current signal, which is proportional to the current flowing through the potential output; and an analyzing unit connected to receive a signal corresponding to the voltage at the potential output, the signal sent by the summing unit at the output thereof and the signal sent by the current-measuring device, the analyzing unit being configured to generate an impedance signal from the received signals.

2. A device in accordance with claim 1, wherein the output of the summing unit is connected to the potential output.

3. A device in accordance with claim 2, further comprising a further amplifier is provided, wherein the output of the summing unit is connected to an input of the further amplifier; an output of the further amplifier is connected to the potential output; and the current-measuring device is operatively connected between the output of the further amplifier and the potential output.

4. A device in accordance with claim 1, wherein: the measuring amplifiers have a first and a second input; each measuring input is connected to the first input of the associated measuring amplifier; and the output of the summing unit is connected to the second inputs of the measuring amplifiers.

5. A method for determining the impedance of the connection between an additional electrode and the skin of a patient, the method comprising the steps of: providing a device comprising a plurality of measuring inputs for connecting to measuring electrodes, the measuring electrodes being adapted to be placed on the body of a patient, a plurality of measuring amplifiers, wherein one of the plurality of measuring amplifiers is associated with each measuring input and each measuring input is connected to an input of the associated measuring amplifier, and a potential output for connecting to the additional electrode, wherein the potential output is designed such that a preset voltage can be applied thereto; generating a mean value signal, which is an indicator of a mean value of the level of the signals sent by the measuring amplifiers; applying a preset voltage to the potential output; providing a voltage signal, which is an indicator of the level of the voltage, which is applied to the potential output; detecting current flowing through the potential output; generating a current signal, which is an indicator of the level of the current flowing through the potential output; and generating an impedance signal from the mean value signal and the current signal.

6. A method in accordance with claim 5, wherein the mean value signal is applied to the potential output.

7. A method in accordance with claim 6, wherein the mean value signal is amplified, and the amplified signal is fed to the potential output.

8. A method in accordance with claim 5, wherein: the measuring amplifiers have a first input and a second input; each measuring input is connected to a first input of the associated measuring amplifier associated; and the mean value signal is fed to the second inputs of the measuring amplifiers.

9. A method for determining an impedance of a connection between an additional electrode and skin of a patient, the method comprising the steps of: providing a device comprising a plurality of measuring inputs for connecting to measuring electrodes, said measuring inputs receiving electrical signals of the patient from the measuring electrodes, said device including a plurality of measuring amplifiers, each of said measuring amplifiers receiving the electrical signals from one of said measuring inputs and generating an output electrical signal proportional to the received electrical signal, wherein one of said plurality of measuring amplifiers is associated with each said measuring input and each said measuring input is connected to an input of said associated measuring amplifier, said device including a potential output configure to connect to, and apply a voltage to, the additional electrode; generating a mean value signal which is a mean value of the output signals generated by said measuring amplifiers; applying the voltage to said potential output; providing a voltage signal, which is an indicator of a value of the voltage which is applied to said potential output; detecting current flowing through said potential output; generating a current signal which is an indicator of the magnitude of the current flowing through said potential output; and generating an impedance signal from the mean value signal and the current signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings:

(2) FIG. 1 is a schematic view of a first exemplary embodiment of a device according to the present invention; and

(3) FIG. 2 is a schematic view of a second exemplary embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) Referring to the drawings, FIG. 1 shows the first exemplary embodiment of a device 1 according to the present invention, wherein the device 1 is connected to a patient 3. The capacities C.sub.1, C.sub.2, C.sub.3 and C.sub.4 as well as the power supply 5 suggest in this connection that both the patient 3 and the housing 7 of the device 1 are coupled capacitively to the surrounding area and especially to a 50 Hz or 60 Hz alternate voltage field, so that the patient 3 and the housing 7, which forms the device ground of the device 1, lie on potentials that are undetermined and different from one another.

(5) The device 1 according to the present invention has a plurality of measuring inputs 9, via which the device can be connected to electrodes 11 on the patient 3, so that a potential on the skin of the patient 3 can be measured via the electrodes 11. The impedances Z.sub.1, Z.sub.N suggest in this connection that the coupling between the electrodes 11 and the skin of the patient 3 is connected to an impedance. However, the present invention is not limited to the use in electrodes, which are placed on the skin of a patient, but rather may also be used, for example, in invasively applied electrodes.

(6) Furthermore, the device 1 has measuring amplifiers Op.sub.1, . . . , Op.sub.N, which have a first, non-inverting input characterized by + and a second, inverting input characterized by . In the exemplary embodiment described here, the measuring amplifiers Op.sub.1, . . . , Op.sub.N are connected as non-inverting amplifiers in reference to the input signals V.sub.1, . . . , V.sub.N by the measuring inputs 9. However, they may also be connected as inverting amplifiers, wherein this must then, however, be taken into consideration in the further processing of the output signals E.sub.1, . . . , E.sub.N of the measuring amplifiers Op.sub.1, . . . , Op.sub.N.

(7) The outputs 13 of the measuring amplifiers Op.sub.1, . . . , Op.sub.N are connected to a microprocessor unit 15, which is designed by program implementation such that the following functions or units are implemented by it.

(8) While the units explained below in the preferred exemplary embodiment described here are implemented by a digitally working microprocessor unit, it is just as possible for these units or functions to be implemented by analog technique, for example, to implement them by means of operational amplifiers. This is likewise covered by the scope of the present invention.

(9) First, a summing unit 17 is implemented by the microprocessor unit 15, which is configured such that the microprocessor unit 15 is connected to the outputs 13 of the measuring amplifiers Op.sub.1, . . . , O.sub.N, and is likewise coupled to these outputs 13. The microprocessor unit 15 sends a signal to an output 19, which corresponds to the mean value of the output signals E.sub.1, . . . , E.sub.N sent by the measuring amplifiers Op.sub.1, . . . , Op.sub.N in this exemplary embodiment, i.e., the signal is either equal to the mean value or proportional to the mean value.

(10) Furthermore, the microprocessor unit 15 is designed such that the mean value signal, which is generated at the output 19 of the summing unit 17, is fed to an output 23 via a detection unit 21. The output 23 is connected to a potential output 27 of the device 1 via a first amplifier (further amplifier) 25.

(11) Between the output of the first amplifier 25 and the potential output 27, the device 1 has another, second amplifier (another amplifier) 29 in the preferred exemplary embodiment described here, which is connected as a current-voltage converter, so that a signal is generated at its output, which signal corresponds to the current between the output of the first amplifier 25 and the potential output 27. Thus, the second amplifier 29 works as a current-measuring device, and the output signal thereof is fed to the microprocessor unit 15.

(12) The potential output 27 of the device 1 is connected to a common or additional electrode 31, which is likewise placed on the skin of the patient 3, wherein the contact between the additional electrode 31 and the skin of the patient 3 has an impedance Z.sub.C, which shall be monitored as continuously as possible in order to detect when, for example, the additional electrode 31 is detached from the skin of the patient 3.

(13) Moreover, the mean value signal is sent via another output 33 by the microprocessor unit 15 and is fed from there to the second, inverting inputs of the measuring amplifiers Op.sub.1, . . . , O.sub.N characterized by (minus sign). Thus, the reference amplifier principle known from the state of the art is implemented by this feeding back of the mean value signal.

(14) Finally, an analyzing unit, which is connected, on the one hand, to the detection unit 21 and, on the other hand, to the output of the second amplifier 29, is implemented in the microprocessor unit 15. Thus, the starting signal of the summing unit 17, a signal, which corresponds to the voltage, which is applied to the potential output 27, and by the first amplifier 25, as well as a current signal, which corresponds to the current flowing through the potential output 27, are fed to the analyzing unit 35.

(15) On the basis of these signals, the analyzing unit calculates an impedance signal from the mean value signal and the current signal. This impedance signal may be sent to an output 37 of the device 1 and be further used, for example, for generating an alarm. The impedance signal may be generated such that the current signal and the mean value signal undergo Fourier transformation and the quotients of the Fourier coefficients are determined.

(16) The second exemplary embodiment of a device according to the present invention, shown in FIG. 2, differs from the exemplary embodiment from FIG. 1 only in that the second amplifier 29 in this case is connected together with a measuring resistor Z.sub.X, such that this detects the voltage drop via the measuring resistor Z.sub.X and consequently detects the current through the potential output. Thus, the combination of a second amplifier 29 and measuring resistor Z.sub.X is herewith used as a current-measuring device. Otherwise, the device 1 works analogously to that of FIG. 1.

(17) The following is utilized in both exemplary embodiments. Based on FIG. 2, the equation
V.sub.CM=V.sub.C+V.sub.x+V.sub.add
is obtained for the common mode voltage V.sub.CM and the common mode signal.

(18) V.sub.C is the voltage dropping between the potential output 27 and the patient, for which
V.sub.C=Z.sub.C.Math.I.sub.C
wherein I.sub.C is the current flowing through the potential output 27 and Z.sub.C is the questionable contact impedance. V.sub.X is the voltage dropped at a measuring resistor Z.sub.X for which the equation
V.sub.X=Z.sub.X.Math.I.sub.C
likewise applies. V.sub.add is an additional voltage, which can be applied to the potential output 27. However, in principle, this voltage may also be zero, i.e., V.sub.add=0.

(19) When one combines the first two equations, on the one hand, and utilizes the relationship, the mean value E.sub.l of the starting signals E.sub.1, . . . , E.sub.N corresponds to the common mode voltage, i.e., E.sub.l=V.sub.CM,

(20) $Z_C=\frac{\stackrel{\_}{E_I}-V_X-V_{\mathrm{add}}}{I_C}$
is obtained.

(21) In the exemplary embodiments, I.sub.C is detected by means of the current-measuring device, i.e., the second amplifier 29, either directly or as voltage at the measuring resistor Z.sub.X, and a corresponding signal is fed to the analyzing unit 35. Moreover, E.sub.l or a signal that is proportional to it is determined by the summing unit 17 and a corresponding signal is likewise fed to the analyzing unit 35. Finally, a signal corresponding to V.sub.add is sent by the detection unit 21 to the analyzing unit 35, so that based on the last equation of the analyzing unit 35, an impedance signal is generated and can be sent to the output 37.

(22) The result of the last equation in this connection is that the level of this signal is in any case proportional to the quotient of the mean value signal E.sub.l and the current I.sub.C through the potential output 27, wherein depending on how V.sub.add is selected, this ratio, for example, corresponding to the amplification in the first amplifier 29, must be corrected.

(23) It has been assumed up to now that the additional voltage V.sub.add is a voltage that is proportional to the mean value signal E.sub.l. However, the detection unit 21 may generate an additional voltage signal V.sub.add, which has a known, variable course over time, for example, a sinus-shaped course. Corresponding information is then sent from the detection unit 21 to the information from the analyzing unit 35, so that a corresponding impedance signal may then also be calculated, wherein the above last equation is used here as well.

(24) While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

APPENDIX

List of Reference Numbers

(25) 1, 1 Device 3 Patient 5 Power supply 7 Housing 9 Measuring input 11 Electrode 13 OutputMeasuring amplifier 15 Microprocessor unit 17 Summing unit 19 Outputsumming unit 21 Detection unit 23 Outputmicroprocessor unit 25 First amplifier 27 Potential output 29 Second amplifier 31 Additional electrode 33 Additional output 35 Analyzing unit 37 Output V.sub.i Input signal E.sub.i Output signal Op.sub.i Measuring amplifier