31.4 A 128-Channel 2mmx2mm Battery-Free Neural Dielet Merging Simultaneous Multi-Channel Transmission Through Multi-Carrier Orthogonal Backscatter

With the rapid progress of brain-computer interfaces (BCls), miniaturized wireless implants have been investigated as an alternative to traditional cable-based neural interfaces. Simultaneous massive-channel recording capability is essential to study cellular interconnections and network properties that arise from synchronized cellular activity. Meanwhile, the battery-free techniques based on wireless power transfer (WPT) and backscatter communication help to miniaturize the neural-recording implant by battery eliminating and antenna sharing [1]. To merge a large number of channels into a single battery-free chip, there are two conventional topologies (Fig. 31.4.1, left): 1) The wireless system used multiple analog front-ends (AFEs) for signal acquisition and merges the signals before a single analog-to-digital converter (ADC) [2–4]. Even with shared ADC circuits, it still consumes high power and chip area, such as $50\mu\mathrm{W}$ power/channel and 0 $32\text{mm}^{2}$ area/channel in [4]. 2) Multiple channels are merged before the AFE, and then both the power consumption and chip area can be further reduced by sharing the AFE and ADC. However, the input impedance will be pulled down with increasing multiplexed channel numbers, leading to significant signal attenuation. $\ln$ both conventional structures, the radio sends the merged data to an external reader in real time, where the high throughput leads to power-hungry communication circuits [3–6]. As a result, the prior battery-free neural-recording chips can only support simultaneous recording of less than 16 channels [2–6] or a low bandwidth such as $500\text{Hz}$ [1].