A 256-Channel Actively-Multiplexed µECoG Implant with Column-Parallel Incremental $\Delta\Sigma$ ADCs Employing Bulk-DACs in 22-nm FDSOI Technology

Electrocorticography (ECoG) recording is a non-penetrating electrophysiology technique that achieves a good balance between spatial resolution, brain coverage and invasiveness [1]. For this reason, it is widely used for the diagnosis of neural disorders and holds promise for prosthetic applications. Although ECoG electrodes have been until recently quite large (~4mm diameter), several studies have shown that micro-scale electrodes (< 1mm diameter) are better suited for studying cortical pathology and implementing neural prostheses [2]. The design of flexible, high-density
$\mu\mathsf{ECoG}$
arrays is challenging since every electrode must be individually addressed, resulting in a wiring bottleneck. Therefore, passive µECoG arrays have a limited electrode count and suffer from poor spatial coverage [3]. In contrast, active
$\mu\mathsf{ECoG}$
arrays have emerged to provide higher spatial coverage with better spatial resolution. The most prominent active array [4] uses Si nanomembrane transistors to multiplex the electrodes and significantly reduce the signal routing. However, the source follower in this array severely limits its noise performance. Moreover, multiplexed
$\mu\mathsf{ECoG}$
arrays impose additional design challenges on the readout IC (ROIC): i) compared to regular non-multiplexed ROICs [3], at least N times higher bandwidth (BW) is required (where N:1 is the multiplexing ratio); ii) the electrode DC offsets (EDOs) from N electrodes are modulated and cannot be removed with traditional AC coupling or DC servo loops (DSLs); and iii) sufficiently low noise is needed to compensate for the noise aliasing caused by the electrode multiplexing.