The Digital-to-Analog Converter (DAC) is the building block that converts the purely numeric output of the NCO to a form suitable for external use, usually a voltage or current. The DAC has a D-bit input bus, which in a DDS connects to the D-bit output bus of the Angle-to-Amplitude Converter. A DAC works by treating the binary number at its input as a fraction of its input bus range, that is, a D-bit bus can represent binary numbers from 0 to 2 to the D minus 1. Any particular binary number x in that range represents a fraction of the range in a perfectly linear manner. The DAC output, whether voltage or current, has lower and upper limits: Imin and Imax for current or Vmin and Vmax for voltage. The DAC works by setting its output value to a fraction of its output range in the same proportion as the input value x is a fraction of its input range. That is, for a binary input of 0, the DAC output is Imin or Vmin, as the case may be. Similarly, a binary input of 2 to the D minus 1 (the maximum value) results in Imax or Vmax at the output. Binary values at the input between 0 and 2 to the D minus 1 result in an output proportionally between the minimum and maximum voltage or current values.