Estimate DAC nonlinearity. The SID contains R-2R ladder, and some likely errors in the resistor lengths which result in errors depending on the bits chosen.
This model was derived by Dag Lem, and is port of the upcoming reSID version. In average, it shows a value higher than the target by a value that depends on the _2R_div_R parameter. It differs from the version written by Antti Lankila chiefly in the emulation of the lacking termination of the 2R ladder, which destroys the output with respect to the low bits of the DAC.