In a monitoring device including a reader and a data carrier, the data carrier includes a receiver coil (201), a resonator capacitor (202), rectifier means (203), a modulation capacitor (206), an energy storage capacitor (204), three modulation switches (209, 210, 211) and data processing means. The modulation capacitor (206), the energy storage capacitor (204), the data processing means and the three modulation switches (209, 210, 211) are arranged such that, in a first configuration, the modulation capacitor (206) and the energy storage capacitor (204) are coupled in parallel and, in a second configuration, the modulation capacitor (206) and the energy storage capacitor (204) are coupled in series. The invention further provides a method of operating a data carrier in such a monitoring device.
