A dual-mode, 6-bit asynchronous delta modulator is presented that automatically switches between fixed- and floating-window operation depending on input signal activity to save power. The design is fabricated in 0.18um CMOS. At 1.2V, power consumption is as low as 0.97µW; max input frequency range is up to 2kHz with a peak SNDR of 36.3dB. Due to dual-mode operation, a ECG waveform measurement demonstrates 5X power reduction compared to single-mode operation with minimal accuracy degradation.