Patent ID: 8763559

Claim:
A cage for holding and monitoring a rodent comprising: a bridge of four legs comprising: a first coil positioned below a floor of the cage; and a second coil positioned above the floor of the cage; a resonant circuit attached to a portion of a laboratory animal, said resonant circuit defining a resonant frequency; a signal generator electrically coupled to at least one coil configured to drive the at least one coil with at least a first drive signal comprising a first frequency and a second drive signal comprising a second frequency, wherein the second frequency is more distant from said resonant frequency than the first frequency; and a detection circuit comprising a differential amplifier electrically coupled to at least one coil and configured to detect an impedance change in said at least one coil between application of said first frequency to said at least one coil and application of said second frequency to said at least one coil, and to generate an output signal representative of resonant circuit position with respect to said at least one coil based at least in part on said impedance change; wherein the signal generator is coupled to a first and second node of said bridge, the first and second nodes being opposite nodes and wherein the detection circuit is coupled to a third and fourth node of said bridge, the third and fourth nodes being opposite nodes.