Automated measurement system for 1- Omega standard resistors using a modified Wheatstone bridge

A modified Wheatstone bridge that has four detectors to sense the imbalances in the circuit and a digitally controlled feedback system to balance it automatically with a resolution of a few parts in 10/sup 9/ is described, and its operating principle is briefly explained. The performance of the fully automated measurement system using the bridge is examined. Data were taken with a measuring current of 50 mA through each resistor. The total integration time of 120 s for each feedback cycle corresponds to an effective noise bandwidth of 3.3 mHz, so that the estimated voltage resolution is 35 pV using the experimentally determined noise power spectrum of 0.61 nV/ square root Hz. Combining this with the voltage drop of 50 mV at each resistor, a resolution of 1.43 n Omega is expected for each feedback cycle, and 0.28 n Omega for each relative resistance ratio. The average standard deviation from the linear fits is 0.31 n Omega , which is in good agreement with the estimated value and is only a factor of four larger than the ultimate sensitivity determined by thermal noise of 1- Omega standard resistors.<>