The invention relates to modulation and demodulation circuits, such as envelope detectors used to demodulate amplitude-modulated (AM) signals. By coupling an analog circuit to a port of a digital component, a compact envelope detector can be obtained, which achieves demodulation of AM signals for direct coupling into a digital input port. Accordingly, a compact envelope detector may be used in the data receiving part of a sealed device requiring post-manufacturing data transfer, in combination with additional components that provide electromagnetic coupling, such as inductive, capacitive, or radiative. An example of such a device is a credit card sized authentication token.