A jumper on P1 is used to connect the ring of the BNC connector to either the LEM
reference or the LEM ground. With P1 in the reference position, the 2.5V offset (which is
intended for use with unipolar analog to digital converters) is effectively removed so that
the scope trace is centered naturally at zero volts. When an isolated 5V supply is used
there is no possible ground loop of ± 1.92 A.