Asynchronous, level-crossing ADCs can provide power savings compared to conventional converters for low-activity input signals, valuable for many sensor measurement applications. In this work, we present a hybrid architecture that combines advantages of both fixed-window and floating-window level-crossing approaches. A dual-mode, 6-bit asynchronous delta modulator is presented that automatically switches between fixed-window and floating-window operation depending on input signal activity to save power. The design is fabricated in $0.18 \mu \mathrm{m}$ CMOS process. At a supply voltage of 1.2V, power consumption is as low as $0.97 \mu \mathrm{W}$; maximum full-scale input frequency range is up to 2kHz with a peak SNDR of 36.3dB. As a demonstration of real-world use, a measurement of an ECG input signal while operating in dual-mode demonstrates 5X power reduction compared to single-mode operation with minimal accuracy degradation.