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High Input Impedance op amp

Dear all,

I am measuring the resistance of a large resistor (up to several Mohms) using 4-wire terminal. The resistance is so large that it surpasses the input impedance of my voltmeter (100MOhm). So I am looking for an differential operational amplifier that offers a large input impedance (>2GOhm), that could take input spans several microvolts up to 0.5 volt.

Could you offer me some products that fits into our application?