Active electrodes for EEG and evoked potential

This article reports the design, fabrication and evaluation results of active electrodes. The clinical results demonstrated their reliability and showed that they can be used for all types of biopotential measurements without conductive paste and abrasive skin preparation. A low noise amplifier is packaged on the back of the electrode disk. The laboratory measured performance of electrodes are: input bias current, 1 to 25 pA; input resistance, 1.8 to 3.6 giga-ohms; input capacitance 2 to 4 pF; gain, 0.9996; noise, (input grounded, 0.1 to 100 Hz), 2.4 to 3.4 μV p-p or 0.8 to 1.2 μV-rms; stability (input grounded), input bias stable within 5 μV in 8 hours. These electrodes were evaluated clinically on volunteers at three medical institutions. The results showed that EEG recordings of active electrodes without skin preparation are the same as that of conventional gold cup electrodes with abrasive preparation. Due to the low lead impedance the 60 Hz and other interferences are smaller. Furthermore, because of the very low input bias current, the active electrode is stable with time. Recordings of two EEG electrodes at the time of application (no abrasion) and 48 hours later showed same signal quality. A multi-electrode attachment system is being developed for easy applications