A robust capacitive digital read-out circuit for a scalable tactile skin

This paper presents a robust, capacitive digital readout circuit (ROC) for sensitive skin applications in humanoid robots. The ROC can be calibrated to null the parasitic effects of transducer variation due to physical assembly. A prototype is fabricated in a 130 nm RFCMOS process, with an active area of 221 × 79 μm2 and 1.84 μW power consumption at VDD = 1.2V and 1ms read-out rate. The ROC output is robust to VDD and temperature variations in a range |ΔVDD| ≤ 20% and (25-53)°C. Furthermore, it can provide up to 200mVpp power supply sine wave rejection in the range 50Hz-5MHz at VDD = 1.1V, for an output standard deviation lower than one LSB. Owing to its features and its digital modularity, the ROC was co-designed with a scalable and modular Multi-Walled Carbon NanoTube (MW-CNT) Nano composite transducer, to achieve tunable output sensitivity by adjusting the sensor nominal capacitance and the reference capacitance. The maximum sensitivity of 5.23 fF per LSB was reached when both match. The ROC was then validated with the MW-CNT nanocomposite sensor which exhibits a piecewise behaviour. 5.3 and 7.1 ENOB were extrapolated in the low-load and medium-load regions, respectively. Besides the major advantage of tunable sensitivity, the presented ROC features the lowest acquisition time and one of the most compact sizes among the state-of-the-art ROCs. Moreover, process voltage and temperature (PVT) robust output and ultralow power consumption make this solution very attractive to replicate human physiology at robotic-level.