Differential Hot Electron Injection in an Adaptive Floating Gate Comparator

We introduce differential-mode hot electron injection for adapting and storing analog nonvolatile signed state variables. This approach is compatible with modern digital CMOS technologies and is readily extended to novel circuit applications. We highlight advantages of the technique by applying it to the design of an adaptive floating gate comparator (AFGC). This is the first use of this technique for adaptation in a nonlinear circuit. The AFGC computes appropriate voltages for locally adapting the input floating gate nodes to cancel offsets. The technique is amenable to both local and nonlocal adaptation which allows greater design flexibility. The AFGC has been fabricated in a commercially available 0.35 μm CMOS process. We experimentally demonstrate more than two orders of magnitude reduction in offset voltage: the mean offset is reduced by 416× relative to chips direct from the foundry and by 202× relative to UV-irradiated chips. We consider both static and dynamic adaptation and demonstrate that the the accuracy of dynamic offset cancellation is approximately two orders of magnitude better than static adaptation. In the presence of observed 8% injection mismatch, the AFGC robustly converges to within 728 μV of the desired input offset (mean offset −109 μV, standard deviation 379 μV). Adaptation occurs within milliseconds, with charge retention for more than one month, and variation of offset error with temperature of −15 μV/°C.