We present an ultra-wideband transceiver designed for ultra-low-power communication at sub-10 cm range. The transceiver operates at a 5.6 GHz carrier frequency, chosen to minimize path loss when using a 1 cm² antenna, and can switch its architecture between self-synchronous rectification and low-IF to adapt its power consumption to the channel characteristic in real time. A low-power digital circuit exploits redundancy in the modulation scheme to provide a real-time BER estimate used to close the mode-switching loop. Implemented in 65 nm CMOS, the transceiver consumes 25 μW when transmitting and 245 μW when receiving in low-power mode, plus 45 μW in the clock generator, and only requires an external antenna. Dual-mode operation allows range extension and mitigates interference.