Disclosed is a test apparatus for measuring the common-mode parasitic capacitance between a first element and a second element being isolated from the first element. The test apparatus includes a signal generating device connected to the first element and having an internal signal source connected in series with a first internal impedance for sending a signal to the first element, and a signal receiving device connected between the second element and the first element and having a second internal impedance for measuring a signal response between the first element and the second element, thereby calculating the common-mode capacitance between the first element and the second element based on the signal response.