The impedance values can be recorded in the XDF file format as a separate LSL stream.
If you decide to observe and/or record impedance values together with the EEG signal, the high frequency noise, stemming from impedance measurement, will be embedded in the recorded signal. This noise has a regular frequency of 125 Hz - when sampling at 500 Hz, and 62.5 Hz for 250 Hz sampling frequency. Also, the input referred noise in the impedance recording mode is higher, as the input impedance of the amplifier is decreased from 1 G-Ohms to 500 M-Ohms.
We advise turning off the impedances, and making sure they are low and stable before the recording.
Please note that the sampling frequency for the impedance values is much lower than for the EEG data (impedance sampling frequency in 500 Hz mode is 4 Hz, while in 250 Hz mode is 2 Hz).