In this paper, the tradeoff between conversion time and power in nW-power capacitance-to-digital converters (CDCs) is explored. The CDC in this work leverages the delay-power flexibility of dual-mode logic, is based on swappable oscillators and operates at nW power and low voltage down to 0.3 V without requiring any additional circuitry, reference or voltage regulation. Its self-calibration compensates PVT variations and mismatch at any point of the chip lifecycle, eliminating the need for trimming at testing time. Testchip demonstration of the CDC in 180nm shows that its power consumption can be dynamically adjusted from 1.37 nW down to 418 pW at a conversion time down to hundreds of ms. This makes the CDC suitable for harvested systems with very limited tight power budget and fluctuating voltage.