Current variation, also known as return difference, refers to the deviation between the two results indicated by the same measured variable during the measurement of the upper stroke and the lower stroke of the instrument. According to the relevant public information, the principle of the return difference circuit is to use the flip of the circuit to shape the signal with slow edge change into a rectangular wave with steep edge. If there is no return difference in the circuit, the output waveform will be disturbed at the top and the output will be open.