It was initially presumed that the DC reference of the individual ADCs
was exhibiting a regulation effect as the input signal was biased
positive for position in the GUI. The waveforms 120107a through C show
that the loading effect depends on whether or not the biased input
varies from half-reference levels. At midpoint position in the GUI, no
reference loading occurs, dependant upon the stiffness of reference
decoupling.

http://www.magma.ca/~legg/TVS/120107a_deviation.jpg
http://www.magma.ca/~legg/TVS/120107b_deviation.jpg
http://www.magma.ca/~legg/TVS/120107c_deviation.jpg

A unipolar loading effect could have been compensated fairly simply by
using the ADC outputs themselves to contribute reference bias, as the
ADC output is positive logic and the originally noted reference
regulation was negative. It's possible that the same compensation is
still viable. Will let you know.

RL