A Subthreshold Time-Domain Analog Spiking Neuron With PLL-Based Leak Circuit and Capacitive DAC Synapse

The design and measurement of a time-domain analog spiking neuron is described. The proposed neuron leverages time-domain processing using voltage-controlled oscillators (VCOs) and a time-domain comparator to integrate the input spike and trigger the output spike. A novel leaky circuit uses a phase-locked loop (PLL) to drive the phase difference between the two VCOs toward zero. A weighted capacitive digital-to-analog converter (CDAC) synapse merges the input spikes and phase-frequency detector (PFD) outputs to generate the VCO control voltage. The neuron is implemented in a 28-nm CMOS technology and operates under a subthreshold supply voltage of 0.35 V. Occupying
$154~\mu {\mathrm{ m}}^{2}$
, measurement shows a maximum spike rate of 5.5 MHz and energy consumption of 159 fJ/spike.