A calibration technique for very low current and compact tunable neuromorphic cells:: Application to 5-bit 20-nA DACs

Low current applications, like neuromorphic circuits, where operating currents can be as low as a few nanoamperes or less, suffer from huge transistor mismatches, resulting in around or less than 1-bit precisions. Recently, a neuromorphic programmable-kernel 2-D convolution chip has been reported where each pixel included two compact calibrated digital-to-analog converters (DACs) of 5-bit resolution, for currents down to picoamperes. Those DACs were based on MOS ladder structures, which although compact require 3N + 1 unit transistors (N is the number of calibration bits). Here, we present a new calibration approach not based on ladders, but on individually calibratable current sources made with MOS transistors of digitally adjustable length, which require only N-sized transistors. The scheme includes a translinear circuit-based tuning scheme, which allows us to expand the operating range of the calibrated circuits with graceful precision degradation, over four decades of operating currents. Experimental results are provided for 5-bit resolution DACs operating at 20 nA using two different translinear tuning schemes. Maximum measured precision is 5.05 and 7.15 b, respectively, for the two DAC schemes.