Here is an app note from Linear Technology describing how to minimize switching regulator noise in linear regulator outputs.
Linear regulators are commonly employed to post-regulate switching regulator outputs. Benefi ts include improved stability, accuracy, transient response and lowered output impedance. Ideally, these performance gains would be accompanied by markedly reduced switching regulator generated ripple and spikes. In practice, all linear egulators encounter some diffi culty with ripple and spikes, particularly as frequency rises.