A current inputted from a first input terminal and a reference current generated by an internal reference current generator circuit, which are in phase, are inputted to a reference current selector circuit by which either the inputted current or the reference current is selected. A digital-to-analog converter coupled to the sensor and a digital transmitter and receiver for wireless digital two-way communication with an evaluator station. The processing device is arranged to selectively provide a status indication of the animal in response to a result of the comparison.