R2R DAC

This is a simple R2R DAC circuit:

         Paralel Port:

          signal   pin        20k   20k
          D0       2      >---###-+-###--0v
                              20k # 10k
          D1       3      >---###-|
                              20k # 10k
          D2       4      >---###-|
                              20k # 10k
          D3       5      >---###-|
                              20k # 10k
          D4       6      >---###-|
                              20k # 10k
          D5       7      >---###-|
                              20k # 10k
          D6       8      >---###-|
                              20k # 10k
          D7       9      >---###-|
                                  # 10k  10uF
                                  +-------][----> To amplifier
                                  # 10k
          GND      20     >-------+------------->
                                  0v