Patent ID: 8068905

Claim:
A method comprising: acquiring a physiological signal through an electrode sensor in contact with a location on a patient's body; providing a frequency stable signal; clocking an input of a counter with the frequency stable signal to produce a counter output, accessing a lookup table with the counter output to produce a lookup table output; applying the lookup table output to a digital to analog converter to produce a digital to analog converter output; low pass filtering the digital to analog converter output to produce an impedance test signal comprising a sine wave having a voltage amplitude, V z , and a known frequency which is higher than a frequency range of the physiological signal being monitored; converting the voltage amplitude, V z , into a test current, I e1 ; at the same time as said acquiring, sending the test current, I e1 , from the electrode sensor in contact with the location on the patient's body to produce a test signal having a voltage component, V e1 ; receiving a combined signal comprising the physiological signal and the test signal, wherein the physiological signal has a voltage component, V s1 ; low pass filtering the combined signal to isolate the voltage component, V s1 , of the physiological signal; band pass filtering the combined signal to isolate the voltage component, V e1 , of the test signal and create a band pass filter output; calculating a contact impedance between the electrode sensor and the patient by dividing the band pass filter output by the test current, L e1 ; and simultaneously displaying the physiological signal and the contact impedance.