A multi-modal analog front end (AFE) and ultra-low energy bio-sensing CMOS SoC is presented. System/ circuit techniques enable signal path duty cycles as low as sub-1% and result in a 35μA Photo Plethysmography (PPG) RX Chain - 5X lower than published state of the art - while maintaining overall SNR > 80dBFS. The signal chain is adaptively synchronized by an ultra-low power FSM and includes a 1.3μW 14b 1kSPS SAR A/D. Input signal-aware, real-time data path adaptation is achieved by leveraging on-the-fly algorithms running on an external microcontroller (μC) to further reduce system energy. A programmable, asynchronous capacitive reset amplifier (PARCA) with NEF of 4.8 and dx/dt analog feature extractor demonstrate energy efficient ECG capture. A battery-powered, Bluetooth low energy (BLE) based, wearable platform with simultaneous ECG and PPG acquisition using this AFE has been demonstrated.