Neural implants and human safety: single-fault detection for DC-coupled recording front ends

DC-coupled analogue front ends (AFEs) for neural implants provide a low-area solution. However, removing the coupling capacitor eliminates the intrinsic barrier that protects cortical tissue: a single-fault event, such as gate-oxide breakdown of a low-noise amplifier (LNA) input transistor, can open a direct DC path from the supply rail into the brain. On the stimulation side this hazard is well understood, and single-fault tolerance is enforced by a series DC-blocking capacitor; on the recording side, DC-coupled front ends discard the equivalent safeguard, yet their protection has gone almost unexamined. This paper presents a single-fault detection mechanism that monitors the LNA for the DC imbalance produced by such a failure and disables the amplifier before the resulting fault current can irreversibly damage tissue. The imbalance is encoded in the duty cycle of a current-starved relaxation oscillator and read out as a time-to-digital measurement. Designed in 65 nm, the mechanism resolves a worst-case fault of 6.4 nA across all corners within 0.81 ms - compliant with the ISO~14708-3 limit for an 8533 um2 electrode - opening a broader discussion of safety in DC-coupled recording.

A mixed-signal analogue front-end for brain-implantable neural interfaces using a digital fixed-point IIR filter and bulk offset cancellation

Advances in miniaturised implantable neural electronics have paved the way for therapeutic brain-computer interfaces with clinical potential for movement disorders, epilepsy, and broader neurological applications. This paper presents a mixed-signal analogue front end (AFE) designed to record both extracellular action potentials (EAPs) and local field potentials (LFPs). The feedforward path integrates a low-noise amplifier (LNA) and a successive-approximation-register (SAR) analogue-to-digital converter (ADC), while the feedback path employs a fixed-point infinite-impulse-response (IIR) Chebyshev Type II low-pass filter to suppress sub-mHz components via bulk-voltage control of the LNA input differential pair using two R-2R pseudo-resistor digital-to-analogue converters (DACs). The proposed AFE achieves up to 41.42dB gain, consumes 2.178uA per channel, occupies 0.198mm2 per channel, and supports neural signal monitoring from 0.1Hz to 10kHz with 3.59uVrms input-referred integrated noise.