Impedance (Electrode Site)

The electrode-tissue interface is not purely resistive: for metal sites, charge crosses it through a double layer that behaves largely as a capacitor, so the number depends on the site's area, its metal, its surface texture and any coating. Site impedance depends on frequency: it decreases as frequency rises.

Two effects reach the data directly. The interface generates thermal noise in proportion to the square root of its resistance, so a higher-impedance site starts with a higher noise floor before the amplifier adds anything. And the site's impedance forms a divider with the amplifier's input impedance: when the two are comparable, part of the signal is lost before it is measured, so the amplifier input impedance must be well above the site impedance; the Intan RHD chips in NeuroNexus headstages present 13 MΩ at 1 kHz, more than ten times a 1 MΩ site. The working range follows from site area: a 177 µm² site on a NeuroNexus probe typically reads 200 kΩ to 1 MΩ at 1 kHz, larger sites read lower and smaller sites higher. Where impedance must come down without giving up a small site, the variable is surface rather than area: a coating that increases the effective surface inside the same footprint, which is what Z-Coat is for.

Impedance also bounds stimulation, because it caps the charge a site can pass before the interface voltage reaches the point where electrolysis of water begins; a site that records well and one that stimulates safely are constrained by the same number from two directions. And it is the first diagnostic. A site reading far above its typical value has a broken trace, a contaminated surface or a dried residue; one reading far below has a short. Measuring every site before a session is the first check, and probe setup begins with it; the headstage chip measures impedance in place, so the check is run from the acquisition software.