Impedance measurement system

1. A method of monitoring a condition of a plurality of electrodes used in the measurement of a biological signal, the method comprising:

applying a first test signal having a first frequency to at least one of the plurality of electrodes and concurrently applying a second test signal having a second frequency different from the first frequency to at least one of the plurality of electrodes at a same time during which at least a portion of the first test signal is applied;

capturing the biological signal while applying the first and second test signals and generating an output signal that includes both the measured biological signal and the first and second test signals;

retrieving an output amplitude for each of the first and second test signals from the output signal;

calculating an estimated impedance for each of the plurality of electrodes based on the retrieved output amplitudes of the first and second test signals;

generating a quantitative indication of the condition of the plurality of electrodes based on the calculated estimated impedances for the plurality of electrodes; and

providing the quantitative indication of the condition of the plurality of electrodes to a user.

2. The method of claim 1 , wherein receiving an output amplitude for each of the first and second test signals comprises:

determining the output amplitude for the first test signal by measuring a first amplitude of a first signal component of the output signal corresponding to the first frequency; and

determining the output amplitude for the second test signal by measuring a second amplitude of a second signal component of the output signal corresponding to the second frequency.

3. The method of claim 1 , wherein the biological signal comprises an electrocardiogram (ECG) signal.

4. The method of claim 1 , wherein the first frequency and the second frequency are both less than 0.5 Hz.

5. The method of claim 1 , wherein calculating an estimated impedance for each of the plurality of electrodes comprises, for each electrode, calculating a plurality of estimated impedance values over a measurement timeframe, and wherein the method further comprises, for each electrode, calculating a final estimated impedance based on a combination of the plurality of estimated impedance values to reduce an impact of noise on the final estimated impedance.

6. The method of claim 1 , further comprising, for each of the plurality of electrodes:

comparing the estimated impedance for the electrode with a threshold impedance; and

generating an alert separate from the quantitative indication to the user indicating a problem with the electrode when the estimated impedance for the electrode exceeds the threshold impedance.

7. The method of claim 1 , wherein the plurality of electrodes comprise two electrodes, and wherein applying the first and second test signals comprises applying the first test signal to a first electrode and applying the second test signal to a second electrode.

8. The method of claim 1 , wherein the plurality of electrodes comprises at least three electrodes, wherein the at least three electrodes comprise a reference electrode and at least two remaining electrodes, and wherein applying the first and second test signals comprises applying the first test signal to the reference electrode and applying the second test signal to each of the at least two remaining electrodes.

9. A system for monitoring a condition of a plurality of electrodes used in the measurement of a biological signal, the system comprising:

a circuit configured to:

apply a first test signal having a first frequency to at least one of the plurality of electrodes and concurrently apply a second test signal having a second frequency different from the first frequency to at least one of the plurality of electrodes at a same time during which at least a portion of the first test signal is applied;

capture the biological signal while applying the first and second test signals and generate an output signal that includes both the measured biological signal and the first and second test signals;

retrieve an output amplitude for each of the first and second test signals from the output signal;

calculate, for each electrode, a plurality of estimated impedance values over a measurement timeframe based on the retrieved output amplitudes of the first and second test signals; and

calculate, for each electrode, an estimated impedance based on a combination of the plurality of estimated impedance values for the electrode.

10. The system of claim 9 , wherein the circuit is configured to retrieve the output amplitude for each of the first and second test signals by:

determining the output amplitude for the first test signal by measuring a first amplitude of a first signal component of the output signal corresponding to the first frequency; and

determining the output amplitude for the second test signal by measuring a second amplitude of a second signal component of the output signal corresponding to the second frequency.

11. The system of claim 9 , wherein the biological signal comprises an electrocardiogram (ECG) signal.

12. The system of claim 9 , wherein the first frequency and the second frequency are both less than 0.5 Hz.

13. The system of claim 9 , wherein the circuit is configured to:

generate a quantitative indication of the condition of the plurality of electrodes based on the calculated estimated impedances for the plurality of electrodes, the quantitative indication for each electrode comprising a value indicative of a relative quality of an electrode connection based on comparison of the calculated estimated impedance to at least one of:

an initial impedance value measured at application of the electrode; or

a reference impedance value associated with a predetermined quality of the electrode connection; and

provide the quantitative indication of the condition of the plurality of electrodes to a user.

14. The system of claim 13 , wherein the circuit is configured to, for each of the plurality of electrodes:

compare the estimated impedance for the electrode with a threshold impedance; and

generate an alert separate from the quantitative indication to the user indicating a problem with the electrode when the estimated impedance for the electrode exceeds the threshold impedance.

15. A system for measuring an electrocardiogram (ECG) signal, the system comprising:

a plurality of electrodes; and

a circuit comprising a microprocessor and a DC amplifier, wherein the circuit is configured to:

apply a first test signal having a first frequency to at least one of the plurality of electrodes and concurrently apply a second test signal having a second frequency to at least one of the plurality of electrodes at a same time during which at least a portion of the first test signal is applied, wherein both the first frequency and the second frequency are below a frequency range associated with the ECG signal;

capture the ECG signal while applying the first and second test signals and generate, using the DC amplifier, an output signal that includes both the measured ECG signal and the first and second test signals;

retrieve an output amplitude for each of the first and second test signals from the output signal; and

calculate an estimated impedance for each of the plurality of electrodes based on the retrieved output amplitudes of the first and second test signals.

16. The system of claim 15 , wherein the circuit is configured to retrieve an output amplitude for each of the first and second test signals by:

determining the output amplitude for the first test signal by measuring a first amplitude of a first signal component of the output signal corresponding to the first frequency; and

determining the output amplitude for the second test signal by measuring a second amplitude of a second signal component of the output signal corresponding to the second frequency.

17. The system of claim 15 , wherein the circuit is configured to generate a quantitative indication of the condition of the plurality of electrodes based on the calculated impedances for the plurality of electrodes, the quantitative indication for each electrode comprising a value indicative of a relative quality of an electrode connection based on a comparison of the calculated estimated impedance to at least one of:

an initial impedance value measured at application of the electrode, or

a reference impedance value associated with a predetermined quality of the electrode connection.

18. The system of claim 17 , wherein the circuit is configured to:

compare the estimated impedance for the electrode with a threshold impedance;

generate an alert separate from the quantitative indication to a user indicating a problem with the electrode when the estimated impedance for the electrode exceeds the threshold impedance; and

provide the quantitative indication of the condition of the plurality of electrodes and the alert to the user.

19. The system of claim 17 , further comprising, a display device configured to:

display a visual representation of the ECG signal; and

display the quantitative indication of the condition of the plurality of electrodes.

20. The system of claim 17 , wherein the value comprises at least one of a numerical value or a percentage calculated based on the comparison of the calculated estimated impedance to the at least one of the initial impedance value or the reference impedance value.