In this method, an out-of-band ac signal is used for excitation. The ac signal is generated by alternatively providing an internal current source and current sink at the input with a fixed frequency. The excitation frequency is a function of the output data rate and is fDR / 4. This out-of-band excitation signal is passed through the channel and measured at the output.
Sensing of the ac signal is done by passing the signal through the channel to digitize it and measure at the output. The ac excitation signals are introduced at a frequency that is above the band of interest, generating an out-of-band differential signal that can be filtered out separately and processed. By measuring the magnitude of the excitation signal at the output spectrum, the lead-off status can be calculated. Therefore, the ac lead-off detection can be accomplished simultaneously with the ECG signal acquisition.