A digital oscilloscope comprises a sampling unit configured to sample an input signal received from an oscilloscope probe to produce a first stream of digital samples, a first acquisition system configured to store and process the stream of digital samples to produce a first data set, a second acquisition system configured to store and process the first stream of digital samples independent of the first acquisition system to produce a second data set, and a display system configured to concurrently display the first data set in a first format and the second data set in a second format different from the first format.

A voltage detecting device detects a differential voltage between two input nodes having a non-zero common mode voltage. A differential voltage detecting circuit 5 detects the differential voltage by sampling each voltage of the input nodes and outputs a detection voltage indicating a detection result. A leak canceling circuit generates a compensating current, which flows in opposition to a leak current flowing out from the input nodes in correspondence to an operation of the differential voltage detecting circuit. An operation control part controls the leak canceling circuit to perform or stop a canceling operation. A failure diagnosing part performs a failure diagnosis about the leak canceling circuit based on a first detection value and a second detection value of the detection voltages, which are detected during periods when the leak canceling circuit performs and stops the canceling operation.