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Cardiac methods and devices that separate signals using at least two composite signals acquired at least at two input impedances. A target source impedance may be selected, and a cardiac signal may be separated from composite signals using the selected target source impedance. Medical systems includ

Cardiac methods and devices that separate signals using at least two composite signals acquired at least at two input impedances. A target source impedance may be selected, and a cardiac signal may be separated from composite signals using the selected target source impedance. Medical systems include a cardiac device having a housing that provides amplification circuitry configured to have a first amplifier input impedance and a second amplifier input impedance, such as using two separate circuits or switching between two input impedances. One or more electrode assemblies are coupled to the amplification circuitry. A signal processor is provided in the housing configured to separate a source signal using a first composite signal detected at the first input impedance and a second composite signal detected at the second input impedance. The phase response of the first input amplifier circuit is about equal to that of the second input amplifier circuit.

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What is claimed is: 1. A cardiac monitoring device for monitoring a heart, comprising: a housing; amplification circuitry provided in the housing, the amplification circuitry configured to have a first amplifier input and a second amplifier input, the first amplifier input having a first input impe

What is claimed is: 1. A cardiac monitoring device for monitoring a heart, comprising: a housing; amplification circuitry provided in the housing, the amplification circuitry configured to have a first amplifier input and a second amplifier input, the first amplifier input having a first input impedance and the second amplifier input having a second input impedance different from the first input impedance; a first electrode arrangement coupled to the first amplifier input and suitable for coupling to the heart with a first electrode interface impedance; a second electrode arrangement coupled to the second amplifier input and suitable for coupling to the heart with a second electrode interface impedance; and a signal processor provided in the housing and coupled to the amplification circuitry, the signal processor configured to separate a cardiac source signal from another physiological source signal using a first composite signal detected at the first input impedance and a second composite signal detected at the second input impedance, the cardiac source signal having a first source impedance associated therewith, and the another physiological source signal having a second source impedance associated therewith different from the first source impedance; wherein the first electrode interface impedance is smaller than the first and second source impedances, and the second electrode interface impedance is smaller than the first and second source impedances. 2. The device of claim 1, wherein the second input impedance is adjustable relative to the first input impedance. 3. The device of claim 1, wherein the first input comprises a first input amplifier circuit and the second input comprises a second input amplifier circuit, further wherein a phase response of the first input amplifier circuit is about equal to that of the second input amplifier circuit. 4. The device of claim 1, wherein the first electrode arrangement and the second electrode arrangement are operable at a separation distance of about 2 centimeters or less. 5. The device of claim 1, comprising a lead coupled to the housing, wherein the first electrode arrangement and the second electrode arrangement are located on the lead. 6. The device of claim 5, wherein the first electrode arrangement comprises at least one bipolar electrode arrangement. 7. The device of claim 1, the housing comprising an electrode arrangement in or on the housing. 8. The device of claim 1, wherein the first electrode arrangement comprises at least one electrode arrangement configured for subcutaneous placement in a patient. 9. The device of claim 1, wherein the first electrode arrangement comprises at least one electrode array configured for subcutaneous placement in a patient. 10. The device of claim 1, wherein the first electrode arrangement comprises at least one surface electrode arrangement. 11. The device of claim 1, wherein the first electrode arrangement comprises at least one intracardiac electrode arrangement. 12. The device of claim 1, wherein the second input impedance attenuates the cardiac source signal component of the second composite signal relative to the cardiac source signal component of the first composite signal. 13. The device of claim 12, wherein the second input impedance attenuates the cardiac source signal component of the second composite signal by a factor between one-fourth and three-fourths relative to the cardiac source signal component of the first composite signal. 14. The device of claim 1, wherein the another physiological source signal comprises a skeletal source signal. 15. A cardiac monitoring device, comprising: a housing; amplification circuitry provided in the housing, the amplification circuitry including a first amplifier having a first amplifier input and a second amplifier having a second amplifier input, the first amplifier input having a first input impedance and the second amplifier input having a second input impedance different from the first input impedance; a first electrode arrangement coupled to the first amplifier input; a second electrode arrangement coupled to the second amplifier input; and a signal processor provided in the housing and coupled to the amplification circuitry, the signal processor configured to separate a source signal using a first composite signal detected at the first input impedance and a second composite signal detected at the second input impedance; wherein each of the first and second amplifiers comprises a sample and hold amplifier. 16. The device of claim 15, wherein the device comprises a multiplexer coupled to outputs of the first and second amplifiers and to the signal processor. 17. The device of claim 15, wherein the respective sample and hold amplifiers sample the first and second composite signals substantially synchronously. 18. The device of claim 15, wherein the electrode arrangement comprises intracardiac electrodes, and the respective sample and hold amplifiers sample the first and second composite signals substantially synchronously at a sampling frequency greater than about 400 Hz. 19. The device of claim 15, wherein the electrode arrangement comprises subcutaneous, non-intracardiac electrodes, and the respective sample and hold amplifiers sample the first and second composite signals substantially synchronously at a sampling frequency greater than about 50 Hz. 20. The device of claim 15, wherein the second input impedance attenuates the source signal component of the second composite signal relative to the source signal component of the first composite signal. 21. The device of claim 20, wherein the second input impedance attenuates the source signal component of the second composite signal by a factor between about one-fourth and three-fourths relative to the source signal component of the first composite signal. 22. A cardiac monitoring device for monitoring a heart, comprising: a housing; amplification circuitry provided in the housing, the amplification circuitry configured to have a first impedance circuit and a second impedance circuit, the first impedance circuit having a first impedance and the second impedance circuit having a second impedance different from the first impedance; and means for separating a cardiac source signal from another physiological source signal using a first composite signal detected at the first impedance and a second composite signal detected at the second impedance, wherein the second impedance attenuates the cardiac source signal component of the second composite signal relative to the cardiac source signal component of the first composite signal. 23. The device of claim 22, comprising means for switching between the first and second impedance circuits. 24. The device of claim 23, comprising means for filtering the first composite signal and the second composite signal, the filtering means configured to remove frequencies associated with the switching means from the first composite signal and the second composite signal. 25. The device of claim 23, comprising means for sampling the first and second composite signals substantially synchronously at a time when the first and second composite signals are valid to remove frequencies associated switching. 26. The device of claim 22, comprising means for filtering the first composite signal and the second composite signal. 27. The device of claim 22, comprising means for synchronously sampling the first and second composite signals. 28. The device of claim 22, comprising means for changing the impedance of one or both of the first impedance circuit and the second impedance circuit. 29. The device of claim 22, wherein the separating means uses a target source impedance to separate the source signal. 30. The device of claim 22, wherein the second impedance attenuates the cardiac source signal component of the second composite signal by a factor between about one-fourth and three-fourths relative to the cardiac source signal component of the first composite signal. 31. The device of claim 22, further comprising an electrode arrangement coupled to the amplification circuitry and suitable for coupling to the heart with an electrode interface impedance, wherein the cardiac source signal has a first source impedance associated therewith and the another physiological source signal has a second source impedance associated therewith, and wherein the electrode interface impedance is smaller than the first and second source impedances. 32. The device of claim 22, wherein the another physiological source signal comprises a skeletal source signal.