Are you trying to build a simple OpAmp summer ? If you don't need
the hi Z afforded by the PGAs you can always eliminate them. And
take your gain individulally for each channel by picking specific input
resistors. Also offset the OpAmp so circuit works with - values.
http://www.ecircuitcenter.com/circuits/opsum/opsum.htm
Right now your circuit has a gain of - 1 in the OpAmp summer, referenced to ground,
hence will not work, eg. output of OpAmp cannot swing below ground w/o you offsetting
the NI input from something higher than Vss.
Note dropping PGAs has the additional advantage of less offset and drift. If
you still need super high Z inputs, have to use PGA, you still have to offset the
OpAmp.
Regards, Dana.
