This article discusses system-level techniques to optimize the power-performance trade-off in subthreshold circuits and presents a uniform platform for implementing ultra-low power power-scalable analog and digital integrated circuits. The proposed technique is based on using subthreshold source-coupled or current-mode approach for both analog and digital circuits. In addition to possibility of operating with ultra-low power dissipation, because of similar basis for constructing analog and digital parts, a common power management unit could be used for optimizing the power-performance of the entire mixed-signal system. Some circuit examples have been provided to show the performance of the proposed circuits in practice.