Elimination of line impedance error in a three-wire probe interface

Apparatus for providing a signal manifestation of the resistance value of a resistance temperature device (RTD), comprising:

• interconnection means having three electrically conductive lines including a source excitation line connected at one end to one side of the RTD, and including a source return line and a sensed signal line each connected at one end to the other side of the RTD;
• excitation signal source means, connected with selected polarity between the other ends of said source excitation line and said source return line for providing an excitation current signal therethrough to the RTD;
• bias signal source means, connected with said same selected polarity between the other end of said sensed signal line and said other end of said source return line, for providing a bias current signal therethrough at a magnitude and phase equal to that of said excitation current signal;
• and
• amplifier means, having first and second inputs connected, respectively, to said other ends of said source excitation line and said sensed signal line, for providing as the manifestation of the RTD resistance value an output voltage signal at a magnitude proportional to the difference magnitude between the voltage signals appearing at said first and said second inputs in response to said excitation current signal and said bias current signal.