A low voltage interface which in use couples a direct current power source to an alternating current signal diagnostic system, the DC power source comprises a positive node, the AC signal diagnostic system comprises a diagnostic signal output node which applies an AC signal to the DC power source, the AC signal diagnostic system further comprises a measurement input node, the low voltage interface comprising: an interface input node adapted to be electrically coupled to the positive node of the DC power source; a first interface output node adapted to be electrically coupled to the measurement input node of the AC signal diagnostic system; and a voltage-reduction circuit adapted to be electrically coupled to the interface input node and the first interface output node, the voltage-reduction circuit configured to: receive an input voltage from the DC power source via the interface input node, the input voltage comprising a DC component and an AC component; reduce the DC component of the input voltage without affecting a magnitude or phase of the AC component of the input voltage, or reduce the DC component of the input voltage with a constant amplification or a constant phase shift of the AC component of the input voltage, which provides a measurement voltage including a reduced DC component; and provide the measurement voltage to the measurement input node of the AC signal diagnostic system via the first interface output node, wherein the voltage-reduction circuit comprises a DC voltage source electrically coupled in series between the interface input node and the first interface output node, and characterized in that the voltage-reduction circuit is configured to maintain a zero or constant phase between the interface input node and the first interface output node for all the range of AC signals having a frequency between 0.01 Hz and 5000 Hz required by the AC signal diagnostic system. an interface input node adapted to be electrically coupled to the positive node of the DC power source; a first interface output node adapted to be electrically coupled to the measurement input node of the AC signal diagnostic system; and a voltage-reduction circuit adapted to be electrically coupled to the interface input node and the first interface output node, the voltage-reduction circuit configured to: receive an input voltage from the DC power source via the interface input node, the input voltage comprising a DC component and an AC component; reduce the DC component of the input voltage without affecting a magnitude or phase of the AC component of the input voltage, or reduce the DC component of the input voltage with a constant amplification or a constant phase shift of the AC component of the input voltage, which provides a measurement voltage including a reduced DC component; and provide the measurement voltage to the measurement input node of the AC signal diagnostic system via the first interface output node, receive an input voltage from the DC power source via the interface input node, the input voltage comprising a DC component and an AC component; reduce the DC component of the input voltage without affecting a magnitude or phase of the AC component of the input voltage, or reduce the DC component of the input voltage with a constant amplification or a constant phase shift of the AC component of the input voltage, which provides a measurement voltage including a reduced DC component; and provide the measurement voltage to the measurement input node of the AC signal diagnostic system via the first interface output node, wherein the voltage-reduction circuit comprises a DC voltage source electrically coupled in series between the interface input node and the first interface output node, and characterized in that the voltage-reduction circuit is configured to maintain a zero or constant phase between the interface input node and the first interface output node for all the range of AC signals having a frequency between 0.01 Hz and 5000 Hz required by the AC signal diagnostic system.