This paper describes an open source analogue to digital converter designed to meet the requirements of power system waveform measurement. In particular, attention is given to ensuring that waveforms are synchronously sampled to maintain the relationship between the various observed quantities, and the system is synchronized to the UTC time base by means of a GPS receiver, making a wide area comparison of data and synchrophasor estimation possible. This ADC can be used as a front end in Phasor Measurement Unit (PMU) devices, amongst other applications. An advantage of this method is that the sampling clock is disciplined to the UTC time base to ensure an integer number of samples are taken per second at the desired sampling rate. The certainty of the timing of the sample data makes a wide area comparison of the data possible, providing an enabling technology for Waveform Measurement Units (WMU). An open design philosophy is used throughout, with the intention of providing visibility throughout the operation of the device, so that the end user can study its operation and evaluate and modify it within their application.