Voltage sensing system with input impedance balancing for electrocardiogram (ECG) sensing applications

• US 6,208,888 B1
• Filed: 02/03/1999
• Issued: 03/27/2001
• Est. Priority Date: 02/03/1999
• Status: Expired due to Term

• Current Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Original Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Inventors: Yonce, David J.
• Primary Examiner(s): EVANISKO, GEORGE ROBERT
• Application Number:
  US09/243,265
• Time in Patent Office: 783 Days
• Field of Search: 600/509, 128/902, 128/905, 128/908
• US Class Current: 600/509
• CPC Class Codes: A61B 5/30 Input circuits therefor
• CPC Class Codes: A61B 5/305 Common mode rejection
• CPC Class Codes: A61B 5/308 for electrocardiography [ECG]
• CPC Class Codes: H03F 2200/261 Amplifier which being suitable for instrumentation applications
• CPC Class Codes: H03F 3/45475 using IC blocks as the active amplifying circuit
• CPC Class Codes: H03F 3/45932 by using feedback means H03F3/45968 takes precedence
• CPC Class Codes: Y10S 128/901 Suppression of noise in electric signal
• CPC Class Codes: Y10S 128/902 Biological signal amplifier

Abstract

A voltage sensing system includes input impedance balancing for electrocardiogram (ECG) sensing or other applications, providing immunity to common-mode noise signals while capable of use with two electrodes. Signals are received at first and second electrodes having associated impedances. An impedance circuit includes a feedback controller that adjusts an effective impedance associated with the second electrode based on a difference signal, a common mode signal, a phase-shifted (e.g., quadrature common mode) signal, and an impedance associated with the first electrode. As a result, signals associated with each electrode undergo a similar degree of gain/attenuation and/or phase-shift. This reduces common mode noise and enhances the signal-to-noise characteristics of a desired ECG or other output signal, without requiring the use of more than two electrodes.

Claims

1. An apparatus, comprising:

a first input node adapted for receiving a first input voltage;

a second input node adapted for receiving a second input voltage;

an impedance element having an adjustable impedance and including a first and second terminal, the first terminal coupled to the second input node;

a first amplification circuit coupled to the first input node and coupled to the second terminal, the first amplification circuit including a first output node adapted for providing a differential output signal based on a difference between the first input voltage and the second input voltage;

an averager circuit coupled to the first input node and coupled to the second terminal, the averager circuit including a second output node adapted for providing a common mode (CM) output signal based on the first input voltage and the second input voltage; and

an impedance circuit coupled to the first output, coupled to the second output, and coupled to the impedance element, the impedance circuit adapted for adjusting the impedance element based on the differential output signal and the CM output signal.

2. The apparatus of claim 1, further including a phase-shifter circuit, having an input coupled to the second output node and an output coupled to the impedance circuit and providing a phase-shifted CM output signal.

3. The apparatus of claim 2, wherein the phase-shifter circuit includes one of an integrator and a differentiator.

4. The apparatus of claim 2, wherein the impedance circuit includes a feedback controller circuit, the feedback controller circuit including:

a first mixer, coupled to receive the differential output signal from the first amplification circuit and the CM signal from the averager circuit, and providing a first mixer output based on the differential and CM output signals; and

a second mixer, coupled to receive the differential output signal from the first amplification circuit and the phase-shifted CM signal from the phase-shifter, and providing a second mixer output based on the differential and phase-shifted CM output signals.

5. The apparatus of claim 4, further including an impedance control subcircuit coupled to the impedance element and coupled to the first output node, and wherein the feedback controller circuit further includes:

a first integrator, having an input coupled to the first mixer output, and providing a first control signal to the impedance control subcircuit; and

a second integrator, having an input coupled to the second mixer output, and providing a second control signal to the impedance control subcircuit.

6. The apparatus of claim 5, wherein the impedance circuit further includes:

a first filter, coupling the differential output signal from the output of the first amplification circuit to the first mixer;

a second filter, coupling the CM signal from the output of the averager circuit to the second mixer; and

a third filter, coupling the phase-shifted CM signal from the output of the phase-shifter circuit to the second mixer.

7. The apparatus of claim 6, wherein the feedback controller circuit further includes:

a first low pass filter, coupling the first mixer output to the input of the first integrator; and

a second low pass filter, coupling the second mixer output to the input of the second integrator.

8. The apparatus of claim 5, further including a ground node relative to the first input voltage and relative to the second input voltage and wherein the impedance element includes a resistive component and a reactive component, and further wherein the impedance control subcircuit includes:

a first voltage-controlled amplifier (VCA), having first, second, and gain-control inputs, and an output, the first input of the first VCA coupled to receive the signal based on the second input voltage, the second input of the first VCA coupled to the ground node, the gain-control input of the first VCA coupled to receive the first control signal from the feedback controller, and the output of the first VCA controlling the resistive component; and

a second VCA, having first, second, and gain-control inputs, and an output, the first input of the second VCA coupled to receive a signal based on the second input voltage, the second input of the second VCA coupled to the ground node, the gain-control input of the second VCA coupled to receive the second control signal from the feedback controller, and the output of the second VCA controlling the reactive component.

9. The apparatus of claim 8, wherein the impedance control subcircuit includes the impedance element, the impedance element including:

a resistor, controlled by the output of the first VCA; and

a capacitor, controlled by the output of the second VCA.

10. The apparatus of claim 1, further including a first input circuit and a first electrode, the first input node coupled to the first input circuit and further wherein the first input circuit is coupled to the first electrode.

11. The apparatus of claim 10, wherein the first input circuit includes an impedance bootstrap circuit.

12. The apparatus of claim 10, further including a ground voltage relative to the first input voltage and relative to the second input voltage, wherein the first input circuit includes:

an amplifier having a first input, a second input, and an output, wherein the first input of the amplifier is coupled to the output of the amplifier, and the output of the amplifier is coupled to the first input of each of the first amplification circuit and the averager circuit;

a first resistor, having first and second terminals, the first terminal of the first resistor coupled to the output of the amplifier;

a second resistor having first and second terminals, the first terminal of the second resistor coupled to the second terminal of the first resistor, the second terminal of the second resistor coupled to the ground voltage;

a third resistor having first and second terminals, the first terminal of the third resistor coupled to the first input of the amplifier, the second terminal of the third resistor coupled to the second terminal of the first resistor; and

a capacitor having first and second terminals, the first terminal of the capacitor coupled to the first input of the amplifier, the second terminal of the capacitor coupled to the ground voltage.

13. The apparatus of claim 10, wherein the first input circuit includes a phase-shifter circuit.

14. The apparatus of claim 13, wherein the phase-shifter circuit includes a phase lead circuit.

15. The apparatus of claim 14, wherein the phase lead circuit includes a parallel resistor and capacitor.

16. The apparatus of claim 10, wherein the first input circuit includes an input protection circuit.

17. The apparatus of claim 16, further including a positive power supply and a negative power supply, wherein the input protection circuit includes:

a resistor, having first and second terminals, the first terminal of the resistor coupled to the first electrode;

a first diode, having an anode and a cathode, the anode of the first diode coupled to the second terminal of the resistor, the cathode of the first diode coupled to the positive power supply; and

a second diode, having an anode and a cathode, the anode of the second diode coupled to the negative power supply, the cathode of the second diode coupled to second terminal of the resistor.

18. The apparatus of claim 1, further including a second input circuit and a second electrode, the second input node coupled to the second input circuit and further wherein the second input circuit is coupled to the second electrode.

19. The apparatus of claim 18, further including a first control node coupled to the impedance circuit and a second control node coupled to the impedance circuit, wherein the second input circuit includes the impedance element, the impedance element including:

a resistor, having first and second terminals, the first terminal of the resistor coupled to the first control node; and

a first capacitor, having a first and second terminal, the first terminal of the capacitor coupled to the second control node.

20. The apparatus of claim 19, wherein the first capacitor is a negative capacitance circuit.

21. The apparatus of claim 19, wherein the first capacitor has a positive-valued capacitance, and further including a second capacitor having a negative capacitance value, the second capacitor coupled to the impedance element.

22. The apparatus of claim 21, wherein a first terminal of the second capacitor is coupled to the second terminal of the first capacitor.

23. The apparatus of claim 19, wherein the second input circuit includes a phase-shifter circuit.

24. The apparatus of claim 23, wherein the phase-shifter circuit includes a phase lead circuit.

25. The apparatus of claim 24, wherein the phase lead circuit includes a parallel resistor and capacitor.

26. The apparatus of claim 19, wherein the second input circuit includes an input protection circuit.

27. The apparatus of claim 26, further including a positive power supply and a negative power supply, wherein the input protection circuit includes:

a resistor, having first and second terminals, the first terminal of the resistor coupled to the second electrode;

a first diode, having an anode and a cathode, the anode of the first diode coupled to the second terminal of the resistor, the cathode of the first diode coupled to a positive power supply; and

a second diode, having an anode and a cathode, the anode of the second diode coupled to the negative power supply, the cathode of the second diode coupled to the second terminal of the resistor.

28. The apparatus of claim 18, wherein the second input circuit includes an amplifier having a first input, a second input, and an output, wherein the second input of the amplifier is coupled to the output of the amplifier, and the output of the amplifier is coupled to the second input of each of the first amplification circuit and the averager circuit.

29. The apparatus of claim 28, wherein the second input circuit further includes the impedance element the impedance element includes:

a resistor, having first and second terminals, the first terminal of the resistor coupled to the impedance circuit, the second terminal of the resistor coupled to the first input of the amplifier; and

a capacitor having first and second terminals, the first terminal of the capacitor coupled to the impedance circuit, the second terminal of the capacitor coupled to the first input of the amplifier.

30. The apparatus of claim 1, further including a ground node relative to the first input voltage and relative to the second input voltage and wherein the averager circuit includes:

an amplifier, having a first input, a second input, and an output providing the CM signal of the averager circuit, the first input of the amplifier coupled to the ground node;

a first resistor, having first and second terminals, the first terminal of the first resistor coupled to the first input of the averager circuit, the second terminal of the first resistor coupled to the second input of the amplifier;

a second resistor having first and second terminals, the first terminal of the second resistor coupled to the second input of the averager circuit, the second terminal of the second resistor coupled to the second input of the amplifier; and

a third resistor having first and second terminals, the first terminal of the third resistor coupled to the second input of the amplifier, the second terminal of the third resistor coupled to the output of the amplifier.

31. An apparatus, the apparatus including:

a first input circuit providing a first input voltage;

a second input circuit providing a second input voltage;

a first amplification circuit providing a differential output signal based on the first and second input voltages;

an averager circuit providing a common mode (CM) output signal based on the first and second input voltages; and

an impedance circuit coupled to the differential output signal and the CM output signal, wherein the impedance circuit is adapted for matching the first input circuit with the second input circuit.

32. The apparatus of claim 31, further including a phase-shifter receiving the CM output signal and providing a quadrature phase-shifted common mode (QCM) output signal, and wherein the impedance circuit includes a feedback controller circuit, the feedback controller circuit includes:

a first mixer, coupled to receive the differential and CM output signals, and providing a first mixer output based on the differential and CM output signals; and

a second mixer, coupled to receive the differential and QCM output signals, and providing a second mixer output based on the differential and QCM output signals.

33. The apparatus of claim 32, further including a controllable component of an impedance in one of the second input circuit and the impedance circuit, and wherein the feedback controller circuit further includes:

a first integrator, having an input coupled to the first mixer output, and providing a first control signal to control the component of an impedance in one of the second input circuit and the impedance circuit; and

a second integrator, having an input coupled to the second mixer output, and providing a second control signal to control the component of an impedance in one of the second input circuit and the impedance circuit.

34. An apparatus, the apparatus including:

a first input circuit providing a first input voltage;

a second input circuit providing a second input voltage;

a first amplifier providing a differential output signal based on the first and second input voltages;

an averager providing a common mode (CM) output signal based on the first and second input voltages; and

a means for approximately matching the first input circuit and the second input circuit based on the differential and CM output signals.

35. An apparatus for sensing signals relative to a voltage at a ground node, the apparatus including:

a first input terminal;

a second input terminal;

a first buffer, including a first buffer input coupled to the first input terminal, and a first buffer output;

a first series impedance between the first buffer input and the first input terminal;

a first shunt impedance between the first buffer input and the ground node;

a second buffer, including a second buffer input coupled to the second input terminal, and a second buffer output;

a second series impedance between the second buffer input and the second input terminal;

a second shunt impedance having a first and second terminal, the first terminal coupled to the second buffer input;

at least one control node coupled to the second terminal; and

an impedance circuit coupled to the at least one control node and providing at least one control signal such that a first gain or phase between the first input terminal and the first buffer input approximately matches a second gain or phase between the second input terminal and the second buffer input.

36. An apparatus for sensing signals, the apparatus including:

a first input terminal;

a second input terminal;

a first buffer, including a first buffer input that is coupled to the first input terminal, and a first buffer output;

a first effective impedance coupled to the first buffer input;

a second buffer, including a second buffer input that is coupled to the second input terminal and a second buffer output;

a second effective impedance coupled to the second buffer input;

a first amplification circuit, coupled to receive the first and second buffer outputs, and providing a differential output signal based thereon;

a second amplification circuit, coupled to receive the first and second buffer outputs, and providing a common mode (CM) output signal based thereon;

a feedback controller circuit, providing at least one control signal to the second effective impedance such that the second effective impedance approximately matches the first effective impedance, wherein the at least one control signal is based on the differential output signal and the CM output signal.

37. The apparatus of claim 36, further comprising a phase-shifter, coupled to receive CM output signal and providing a quadrature common mode (QCM) output signal based thereon, and wherein the at least one control signal is also based on the QCM output signal.

38. An electrocardiogram system comprising:

a first electrode adapted for being coupled to a patient for receiving an electrical first heart activity signal;

a second electrode adapted for being coupled to the patient for receiving an electrical second heart activity signal;

a first amplification circuit, including a first input that is coupled to the first electrode, a second input that is coupled to the second electrode, and an output providing a electrocardiogram (ECG) output signal based on a difference between the first and second heart activity signals, the ECG output signal having components;

an averager circuit, including a first input that is coupled to the first electrode, a second input that is coupled the second electrode, and an output providing a common mode (CM) output signal based on the first and second heart activity signals, the CM output signal having components;

a first phase-shifter circuit, coupled to the output of the averager circuit to receive the CM output signal, and providing a quadrature common mode (QCM) output signal;

an impedance circuit, coupled to receive signals from the outputs of the first amplification, the averager, and the first phase-shifter circuits, the impedance circuit providing a first control signal based on a first multiplication of components of the ECG output signal with components of the CM output signal, the impedance circuit also providing a second control signal based on a second multiplication of components of the ECG output signal with components of the QCM output signal; and

an impedance control subcircuit, coupled to receive the first and second control signals, the impedance control subcircuit providing an effective impedance, coupled to the second electrode, that is adjusted based on an impedance that is coupled to the first electrode.

39. The system of claim 38, further including a second phase-shifter between the first electrode and a first amplifier.

40. A method of detecting first and second input signals, the method comprising:

receiving the first input signal from a first electrode;

receiving the second input signal from a second electrode;

obtaining a difference signal based on the first and second input signals;

obtaining a common mode (CM) signal based on the first and second input signals;

obtaining a quadrature common mode (QCM) signal that is phase-shifted from the CM signal; and

approximately matching at least one of a gain/attenuation or a phase of the second input signal to at least one of a respective gain/attenuation or a phase of the first input signal, based on the difference, CM, and QCM signals.

41. The method of claim 40, wherein matching the gain/attenuation or phase includes adjusting a impedance coupled to the second electrode based on an impedance coupled to the first electrode.

42. The method of claim 40, further comprising phase-shifting the first input signal at the first electrode before obtaining the difference and CM signals.

43. The method of claim 42, further comprising phase-shifting the second input signal at the second electrode before obtaining the difference and CM signals.

44. A method of detecting a voltage between first and second electrodes, the method comprising:

receiving a first input voltage from the first electrode;

receiving a second input voltage from the second electrode;

obtaining a difference signal based on the first and second input voltages;

obtaining a common mode (CM) signal based on the first and second input voltages;

obtaining a quadrature common mode (QCM) signal that is phase-shifted from the CM signal;

multiplying components of the difference signal with components of the CM signals to provide a first control signal;

multiplying components of the difference signal with components of the QCM signal to provide a second control signal; and

adjusting an effective impedance, coupled to the second electrode, based on the first and second control signals and an impedance coupled to the first electrode.

45. The method of claim 44, further comprising phase-shifting the first input voltage at the first electrode before obtaining the difference and CM signals.

46. The method of claim 45, further comprising phase-shifting the second input voltage at the second electrode before obtaining the difference and CM signals.