Abstract：A DC resistance bridge null-detector circuit based on superconducting quantum interferometer (SQUID) is designed. The circuit can be used as a null-detector circuit for measuring low and medium value resistance (0.1~100Ω) in high-precision resistance bridges. By making the SQUID peripheral circuit and optimizing the mutual inductance value of its input coupling coil, the precise measurement of unbalanced current of weak bridge is realized. Compared with the traditional resistance bridge based on the nanovoltmeter null-detector measuring unbalance voltage, its detection sensitivity is improved by an order of magnitude in the low-to-medium resistance measurement range. The 1/f corner frequency of SQUID flux locked loop system which is designed as a null-detector, is about 2Hz and the white noise level of the magnetic flux is about 3.5μΦ0/. And the mutual inductance between input coil and SQUID is as high as 10nH, its equivalent current white noise density 0.7pA/. When it is used as a resistance bridge null-detector to measure 100Ω standard resistance (operating current is 10 mA), the resolution can reach the order of 10-11.
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