Influence of voltmeter input impedance on quantum Hall effect   measurements
  We report on the influence of voltmeters on measurements of the longitudinal resistance in the quantum Hall effect regime. We show that for typical input resistances for standard digital lock-in amplifiers the longitudinal resistance can show a non-zero minimum which might be mistaken for parallel conduction in the doping layer. In contrast to a real parallel conduction the effect disappears when either the current source and ground contact are swapped or the polarity of the B-field is changed. We discuss the influence of input capacitances and stray capacitances on the measurement. The data demonstrates the influence of the voltmeter input impedance on the longitudinal resistance measurement. 
F. Fischer and M. Grayson