Design and evaluation of a low thermal electromotive force guarded scanner for resistance measurements

The design and testing of a low thermal electromotive force guarded scanner, developed to provide completely guarded switching when used with actively guarded resistance bridge networks, is described. The design provides a continuous guard circuit trace on the scanner circuit boards that surrounds the relay contacts and protects the measurement circuit from leakages to ground. Modification to the circuit boards and relays of the guarded scanner are explained. Several tests were developed to evaluate the guarding effectiveness, including isolating sections of the guard circuit to create a potential drop between the main and guard circuits. Calibration of standard resistors using the guarded scanner has shown relative differences less than 1 x 10(-6), 30 x 10(-6), and 150 x 10(-6) for measurements made with and without the guarded scanner at nominal resistances of 1, 10, and 100 G Omega, respectively. The substitution method was used to significantly reduce the relative differences between channels to less than 0.5 x 10(-6), 3 x 10(-6), and 30 x 10(-6) for nominal resistances of 1, 10, and 100 G Omega, respectively. Applications for the guarded scanner in automated direct current measurement systems are presented. [S0034-6748(99)01306-4].