Active (CMOS) electrode array

An active electrode array integrates transistors directly beneath or beside each electrode on a CMOS die, in contrast to a passive array whose electrodes are simply metal pads wired out to remote electronics. The in-pixel circuitry may be as little as an addressing switch (a switch-matrix array) or as much as a per-pixel amplifier and buffer, letting a single die host thousands to tens of thousands of sites.

The payoff is spatial density far beyond what a passive wire-per-electrode array allows, because the read-out is shared in time rather than one dedicated line per site. This is the architecture behind Neuropixels, high-density in-vitro multielectrode arrays, and CMOS ECoG surface grids. The cost is that only a subset of sites is read at any instant, plus switching charge injection, kT/C sampling noise and crosstalk from the shared read-out.

Active arrays make the front-end IC and the electrode array one object, which tightly couples electrode design to circuit design: site impedance, parasitic capacitance and the multiplexer settling budget must all be co-designed, and any yield defect in the silicon can kill a whole region of sites.