A noncoherent 0-16.7 Mb/s ultra-wideband (UWB) receiver using 3-5 GHz subbanded pulse-position modulation (PPM) signaling is implemented in a 90 nm CMOS process. The RF and mixed-signal baseband circuits operate at 0.65 V and 0.5 V, respectively. Using duty-cycling, adjustable bandpass filters, and a relative-compare baseband, the receiver achieves 2.5 nJ/bit at 10-3 BER with -99 dBm best case sensitivity at 100 kb/s. The energy efficiency is maintained across three orders of magnitude in data rate. For data rates less than 10 kb/s, leakage power dominates energy/bit.