Automatic input impedance balancing for electrocardiogram (ECG) sensing applications

• US 6,496,721 B1
• Filed: 04/28/2000
• Issued: 12/17/2002
• Est. Priority Date: 04/28/2000
• Status: Expired due to Fees

• Current Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Original Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Inventors: Yonce, David J.
• Primary Examiner(s): Getzow, Scott M.
• Application Number:
  US09/561,063
• Time in Patent Office: 963 Days
• Field of Search: 600/509, 128/901, 128/902
• US Class Current: 600/509
• CPC Class Codes: A61B 5/30 Input circuits therefor
• CPC Class Codes: A61B 5/308 for electrocardiography [ECG]

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 for sensing a difference between first and second input voltages, the apparatus comprising:

a reference circuit, where the reference circuit includes an input receiving an amplified signal based on first input voltage, and an output providing a reference voltage;

a compensation circuit, where the compensation circuit includes an input receiving an amplified signal based on the second input voltage, and an output providing a compensated voltage based on reference voltage;

a first amplification circuit, where the first amplification circuit includes a first input receiving a signal based on the reference voltage, a second input receiving a signal based on the compensated voltage, and an output providing a differential output signal based on a difference between the signals of the first and second inputs of the first amplification circuit;

an averager circuit, where the averager circuit includes a first input receiving a signal based on the reference voltage, a second input receiving a signal based on the compensated voltage, and an output providing a common mode (CM) output signal based on the signals at the first and second inputs of the averager circuit; and

an impedance circuit, coupled to receive signals from the outputs of the first amplification and averager circuits, the impedance circuit adjusting an impedance of the compensation circuit, based on the signals received from the outputs of the first amplification circuit and the averager cirucit to reduce an impact of the CM output signal on the ouput of the first amplification circuit.

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

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

4. The apparatus of claim 2, in which the impedance circuit includes a feedback controller circuit, where the feedback controller circuit includes:

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 output signal from the phase-shifter, and providing a second mixer output based on the differential and the phase-shifted CM output signals.

5. The apparatus of claim 4, in which 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 an 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, in which the impedance circuit further includes:

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

a second filter, coupling the CM output signal from the output of the phase-shifter circuit to the first mixer; and

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

7. The apparatus of claim 6, in which 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 6, in which the impedance control subcircuit includes:

a first voltage-controlled amplifier (first VCA), having first, second, and gain control inputs, and an output, the first input of the first VCA coupled to receive a signal based on the compensation voltage, the second input of the first VCA coupled to a 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 a resistive component of impedance that is coupled to the compensation voltage; 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 compensation 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 a reactive component of the impedance that is coupled to the compensation voltage.

9. The apparatus of claim 8, in which the impedance control subcircuit includes the impedance that is coupled to the compensation circuit, which includes:

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 comprises a first input circuit coupled between a first electrode and the input of the reference circuit.

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

12. The apparatus of claim 10, in which the first input circuit comprises a first buffer amplifier having a first input, a second input, and an output, wherein the first input of the buffer amplifier is coupled to the output of the buffer amplifier, second input of the buffer amplifier is coupled to the impedance circuit to receive a first constant impedance signal, and the output of the buffer amplifier is coupled to the input of the reference circuit.

13. The apparatus of claim 1, further comprising a second input circuit coupled between a second electrode and the input of the compensation circuit.

14. The apparatus of claim 13, in which the second input circuit comprises the impedance circuit coupled to the second input voltage, which comprises a second buffer amplifier having a first input, a second input, and an output, wherein the first input is coupled to the impedance circuit to receive second constant impedance signal, second input of the buffer amplifier is coupled to the output of the buffer amplifier, and the output of the buffer amplifier is coupled to the input of the compensation circuit.

15. The apparatus of claim 1, in which the reference circuit comprises:

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

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

16. The apparatus of claim 15, in which the reference circuit further comprises the impedance coupled to the first amplified input voltage, which comprises:

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

17. The apparatus of claim 1, in which the compensation circuit comprises:

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 second input of each of the first amplification circuit and the averager circuit;

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

a capacitor, having a first and second terminal, the first terminal of the capacitor coupled to the effective impedance circuit to receive a second control signal, the second terminal of the capacitor coupled to the first input of the amplifier.

18. The apparatus of claim 17, in which the compensation circuit further comprises the impedance coupled to the second amplified input voltage, which comprises:

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

19. An apparatus for sensing a difference between first and second input voltages, the apparatus comprises:

a first input circuit, providing a first buffered input voltage based on a signal received from a first electrode;

a second input circuit, providing a second buffered input voltage based on a signal received from a second electrode;

a reference circuit, providing a reference voltage signal based on the first buffered input voltage;

a compensation circuit, providing a compensated voltage signal based on the reference voltage signal;

a first amplification circuit, providing a differential output signal based on the reference and compensated voltage signals;

an averager circuit, providing a common mode (CM) output signal based on the reference and compensated input voltages; and

an impedance circuit, approximately matching at least one of a gain/attenuation or a phase of the reference circuit and the first electrode to at least one of a respective gain/attenuation or a phase of the compensation circuit and the second electrode based on the differential and CM output signals.

20. The apparatus of claim 19, further comprising 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 that comprises:

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

21. The apparatus of claim 20, in which the feedback controller circuit further comprises:

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

22. 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;

buffering the first input signal to circumvent a dominance of ECG cable capacitance over first input signal;

buffering the second input signal to circumvent the dominance of ECG cable capacitance over the second input signal;

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 between the CM and QCM signals.

23. The method of claim 22, in which matching the gain/attenuation or phase includes adjusting an impedance coupled to the second electrode based on an impedance coupled to the first electrode.

24. A system, comprising:

a first input circuit, providing a first input voltage based on a signal received from a first electrode;

a second input circuit, providing a second input voltage based on a signal received from a second electrode;

an amplification circuit, providing an electrocardiogram (ECG) output signal based on the first and second input voltages;

a right leg feedback electrode, operatively coupled to the amplification circuit to reduce common mode (CM) noise generated at the first and second electrodes; and

a switch, operatively coupled between the amplification circuit and the right leg feedback electrode to selectively turn-off parasitic feedback coming from a right leg feedback electrode cable, when the right leg electrode is not being used.

25. The system of claim 24, wherein the system is an electrocardiogram (ECG) system.

26. The system of claim 24, wherein the first input circuit includes an impedance bootstrap circuit coupled between a first electrode and the amplification circuit.

27. The system of claim 24, wherein the second input circuit includes an impedance bootstrap circuit coupled between a second electrode and the amplification circuit.

28. The system of claim 24, wherein the amplification circuit, including a first input that is coupled to the first electrode to receive a first heart activity signal, a second input that is coupled to the second electrode to receive a second heart activity signal, and an output providing an electrocardiogram (ECG) output signal based on a difference between the first and second heart activity signals.

29. The system of claim 24, further includes a negative amplifier, operatively coupled between the switch and the amplification circuit to invert and amplify the common-mode noise signal.

30. A method of using an electrocardiogram (ECG) system, comprising:

receiving a first input signal from a first electrode;

receiving a second input signal from the second electrode;

disconnecting a right leg drive signal from a right leg ECG cable when a right leg feedback electrode is not being used;

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

displaying the difference signal as ECG output signal.

31. The method of claim 30, further includes:

determining to see if the right leg feedback electrode is being used to reduce common mode noise coming from the first and second electrodes; and

disconnecting the right leg drive signal from the right leg ECG cable based on an outcome of the determination.

32. The method of claim 30, further includes:

sensing to see if the right leg feedback electrode is being used to reduce common mode noise coming from the first and second electrodes; and

disconnecting the right leg drive signal from the right leg ECG cable based on an outcome of the sensing.

33. An apparatus for sensing a difference between first and second input voltages, comprising:

a first amplification circuit, including a first input receiving a signal based on a first input voltage, a second input receiving a signal based on a second input voltage, and an output providing a differential output signal based on a difference between the signals at the first and second inputs of the amplification circuit;

an averager circuit, including a first input receiving a signal based on the first input voltage, a second input receiving a signal based on the second input voltage, and an output providing a common mode (CM) output signal based on the signals at the first and second inputs to the averager circuit;

an automatic gain control circuit, including a first input receiving the common mode output signal from the averager circuit, and an output providing an amplitude regulated common mode signal;

a phase-shifter circuit, operatively coupled to the output of the automatic gain control circuit to receive the amplitude regulated CM output signal, and provide a quadrature common mode (QCM) output signal; and

an impedance circuit, coupled to receive signals from the outputs of the first amplification and the phase-shifter circuits, where the impedance circuit adjusting an impedance, coupled to the second input voltage, based on the signals received from the outputs of the first amplification circuit and the phase-shifter circuit.

34. The apparatus of claim 33, in which the automatic gain control circuit further is comprises:

a voltage controlled amplifier (VCA), including a first input receiving a common mode output signal from the averager, a second input, and an output providing a controlled signal to a phase-shifter circuit;

an RMS-to-DC converter, including an input receiving the controlled signal from the VCA, and an output providing a direct current signal;

an amplifier, including a first input receiving the direct current signal from the RMS-to-DC converter, a second input receiving a reference voltage signal, and an output providing an amplitude regulated signal based on a difference between the reference and the direct current signals; and

an integrator, including an input receiving the amplitude regulated signal from the amplifier and an output providing an integrated signal to the second input of the voltage controlled amplifier.

35. 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 where the CM signal includes a transient response time;

regulating amplitude of the CM signal such that the transient response time is essentially constant over input noise levels;

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

multiplying components of the regulated common mode signal with the difference signal to provide a first control signal;

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

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

36. The method of claim 35, in which matching the gain/attenuation or phase includes 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.

37. A method of using an electrocardiogram to detect first and second input signals, the method comprising:

receiving the first input signal from a first electrode;

receiving the second input signal form 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;

regulating amplitude of the CM signal to keep a transient response time essentially constant for input noise levels;

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

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

38. The method of claim 37, in which matching the gain/attenuation or phase includes adjusting an impedance coupled to the second electrode based on an impedance coupled to the first electrode.