An apparatus for measuring RF voltage from a quadrupole in a mass spectrometer are provided. The apparatus comprises at least one rectifying diode circuit for rectifying the RF voltage of the quadrupole to produce a rectified RF voltage. The apparatus further comprises at least one operational amplifier configured as a current to voltage converter, a negative input of the at least one operational amplifier connected to the output of at least one diode in the at least one rectifying diode circuit, a positive input of the at least one operational amplifier at ground, and an output of the at least one operational amplifier in a feedback look with the negative input, to reduce reverse leakage current from the at least one diode.