Using a Substitution Measurement Topology to Eliminate the Effect of Common Mode Errors in Resistance Measurements used in Temperature Metrology

This article discusses the advantages of using a substitution measurement topology in place of the conventional common-current series connection arrangement employed in bridges or potentiometric resistance instruments working at the highest (sub ppm) accuracies. Most high accuracy resistance instruments pass a common AC or DC current through the device-under-test (DUT) and a reference standard (REF). The ratio of the resistance of the DUT to that of the REF is determined by balancing a bridge circuit or by taking successive measurements of the voltage across the DUT and REF with a highly linear voltmeter. However, both these techniques are prone to the effect of common-mode signals in the bridge circuit or the differential amplifier used with the voltmeter. An alternative measurement topology is discussed in which the DUT and REF are alternately switched into the same measurement point in the instrument in order avoid common mode changes between measurements on the REF and DUT. Unfortunately, this approach significantly increases the performance demands on the current source used to generate the sense current and this would normally result in comparable errors to those from the commonmode problems we are trying to overcome. However, by using a cascode amplifier between the current source and the measurement point and by employing active guarding this pitfall is avoided. This technique is employed in the new microK range of resistance bridges developed for temperature metrology applications. This paper and the performance results show how this technique allows the instruments to take full advantage of the high linearity and speed of the ADC developed for the products in order to achieve sub milli-Kelvin uncertainties. Although this paper discusses the application of the principle to resistance measurement, it can equally be applied to the measurement of other parameters such as capacitance or inductance.