I think it is explained partially on the website.
But here is how it works, a R2R ladder DAC has a pile of precision resistors, and a pile of switches
in the case of the soekris, lv595 which are serial in parallel out shift registers. The output stages of these
devices are designed to drive digital circuits, not resistors. As such they have definitely non-zero impedance.
And each of the 8 output stages (6 are used in the soekris per chip due to layout) do not have...