Patient electrode impedances are known to decay over time. It is necessary to continuously monitor these electrode connections to verify a suitable connection is present. The ADS1291, ADS1292, and ADS1292R lead-off detection functional block provides significant flexibility to the user to choose from various lead-off detection strategies. Though called lead-off detection, this is in fact an electrode-off detection.
The basic principle is to inject an excitation signal and measure the response to find out if the electrode is off. As shown in the lead-off detection functional block diagram in Figure 37, this circuit provides two different methods of determining the state of the patient electrode. The methods differ in the frequency content of the excitation signal. Lead-off can be selectively done on a per channel basis using the LOFF_SENS register. Also, the internal excitation circuitry can be disabled and just the sensing circuitry can be enabled.
NOTE:The RP value must be selected in order to be below the maximum allowable current flow into a patient (in accordance with the relevant specification the latest revision of IEC 60601).