Broadband digital-to-analogue converters (DACs) are indispensable components of modern signal processing systems. In test, measurement, sensing, mm-wave and THz wireless communication, copper-based wireline communication, fiber-optic communication, and many other application areas DACs are used and often the DAC bandwidth and/or resolution represents the limiting factor of the system. Photonic DACs allow for bandwidths far beyond the bandwidth of any electronic DAC. Often photonic DAC concepts use mode-locked-lasers (MLLs) as ultra-broadband optical pulse sources for the photonic DAC where the optical pulse train is modulated with a digital signal. In this way ultra-broadband digitally modulated output signals can be generated. However a disadvantage of this approach is the imprecise bandwidth of the DAC output signal. We argue that a DAC which is to be used in demanding system applications - such as for instance fiber-optic communication links using dense wavelength division multiplexing or wireless communications - does not simply need a maximally large bandwidth but rather a precisely-confined bandwidth. In signal theory an ideal DAC filters a weighted impulse sequence with a rectangular low pass filter whereby the output signal represents a sequence of sinc-shaped pulses. Hence ideally the photonic DAC requires an optical pulse source which generates precise sync-shaped pulses which is difficult to generate with conventional optical pulse sources such as e.g. an MLL. We propose a new Photonic DAC concept, the PONyDAC (Precise Optical Sinc-shaped Nyquist Pulse Synthesizer DAC) which combines precise ultra-broadband Nyquist pulse synthesis with a time-interleaved photonic DAC concept. The concept is fully compatible with silicon photonics integration and does not require an MLL. In system experiments we will demonstrate precise Nyquist pulse synthesis up to 500 GHz bandwidth using discrete laboratory equipment. Furthermore as a proof-of-concept we will implement the PONyDAC in silicon photonics technology taking full advantage of electronic-photonic co-integration of high-speed Silicon Germanium heterobipolar transistors with SOI-based silicon photonics technology.