Energy-Efficient Hybrid Analog/Digital Approximate Computation in Continuous Time

We present a unit that performs continuous-time hybrid approximate computation, in which both analog and digital signals are functions of continuous time. Our 65 nm CMOS prototype system is capable of solving nonlinear differential equations up to 4th order, and is scalable to higher orders. Nonlinear functions are generated by a programmable, clockless, continuoustime 8-bit hybrid architecture (ADC + SRAM + DAC). Digitally assisted calibration is used in all analog/mixed-signal blocks. Compared to the prior art, our chip makes possible arbitrary non-linearities and achieves 16x lower power dissipation, thanks to technology scaling and extensive use of class-AB analog blocks. Typically, the unit achieves a computational accuracy of about 0.5% to 5% RMS, solution times from a fraction of 1 mu s to several hundred mu s, and total computational energy from a fraction of 1 nJ to hundreds of nJ, depending on equation details. Very significant advantages are observed in computational speed and energy (over two orders of magnitude and over one order of magnitude, respectively) compared to those obtained with a modern microcontroller for the same RMS error.