This article describes a scanning probe microscopy technique for quantitative high-speed voltage wave form measurement inside an operating integrated circuit. Internal signals are determined by sensing the local electrostatic force on a noncontacting micromachined probe cantilever that is closely positioned above the circuit test point. Amplitude modulation based downconversion is employed to measure repetitive high-frequency signals which can have a bandwidth much greater than the mechanical response of the probe. A force-nulling technique is used to obtain accurate absolute voltages without the need for complex calibration or precise probe positioning, and enables direct measurement of passivated circuits. A method of eliminating dc offset errors, such as that due to material work function differences, is described. Measurement of signals on the passivated interconnects of a wideband distributed amplifier is presented. The instrument demonstrates a voltage accuracy of <30 mV over a dynamic range of 2.5 V.