Described examples include low power analog front end circuits for sensing repeating signal waveforms, including a first sampling circuit to sample an input signal, an analog detector circuit to provide a detector output signal representing a feature of the input signal, a second sampling circuit to sample the detector output signal, and a control circuit to control a sample rate or other analog front end operating parameter at least partially according to the sampled detector output signal, and to selectively enable and disable the analog detector circuit at least partially according to a model representing an expected repeating waveform of the input signal.