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In retargeting of a nano-watt CMOS reference circuit, we adopt an advanced compact MOSFET model to describe the drain current consistently in strong and weak inversion levels. Based on this model, we describe all bias conditions in terms of ratios of the channel widths and lengths. Taking the effect of very long channels into account, we formulate the threshold voltage as a function of the drain-source voltage. Furthermore, we introduce a tuning parameter with the empirical range and fix all transistor sizes sweeping this parameter as well as applying a simulation. In case studies, we retargeted a circuit from the 180nm/1.8V process to the 90nm/1.2V, 2.5V, 3.3V processes. Besides, we fabricated the circuit in the 90nm/1.2V process, and confirmed the good measurement results such as less than 12.8%/V supply voltage variation and only 1.1nW power consumption.