What are these? Is it like some 'physical ring' where you out a wire through it or just a connection to make both input to the op-amp common? How does this really help? Seems like you're making a source common to both like gnd so if the gnd is noisy it becomes common for both and rejects it? Here's a chip i was considering using, a JFET due to the low input noise. http://focus.ti.com/lit/ds/slos182b/slos182b.pdf Page 64 talks about using guard rings and driving them from a low impedance source at the same voltage level as the common mode input.