Wed May 14 00:54:50 EDT 2014

Diode reference + non-inverting summing amp

Instead of lifting up the whole DAC, it might be simpler to add an
offset to the signal after conversion using a non-inverting summing

The offset to add is about twice the max range, so a 2R / 1R ratio
could do it.  It's definitely a lot less finicky than lifting the
ground.  A nice thing about this is that the reference voltage is a
diode drop, so no hassle with resistors.  The range will be +- ok by
itself so no need to worry about huge current differences.