ADS1299: How to Measure Impedance of Active Electrodes

I am experimenting with TI ADS1299 evaluation kit for EEG acquisition. I see the that ADS1299 has provisions for measuring electrode impedance with the skin to validate the measurement. The electrodes I shall be using are active dry electrodes which have inherent unity gain amplifier. I understand that methods used with 1299, like injecting small currents, are suitable only for passive electrode without electronics and connected directly to ADS1299.  Can You recommend a method / application suitable for measuring electrode to skin impedance with active electrodes which have electronic buffers  inside.