  One of the many limitations with the mixed-signal design is physically
testing circuit ideas. While it is easier to test digital circuits with FPGAs,
this can not be done usually with mixed-signal circuits. Although some FPGAs
have built-in analog-to-digital and digital-to-analog converters, regular
commercial FPGAs development boards and low-cost FPGAs lack built-in data
converters. Here we introduce an all-digital FPGA-based DAC, which is one of
the main blocks to enable mixed-signal experiments. The DAC can be synthesized
entirely in an FPGA and does not require the use of external components.
Furthermore, and to extend its range of applications, a discussion regarding
the proposed DAC's problems and possible solutions is presented. Experimental
demonstration of a 4-bit and a 5-bit DAC corroborate the theoretical analysis
developed in this work. This work also suggests a scheme which includes few
external resistors to improve the linearity (DNL$\leq$0.25LSB and an
INL$\leq$0.5LSB), and the power consumption (5X improvement over the standalone
configuration).
