Weel maybe I'm just a little slow. Mr. Lockhart answered my questioned (who, by the way, happens to have some of the best USB AC Data Aq. for the buck -- a shameless plug for helping me)
Single-ended. An unbalanced input, non isolated. Not suitable for off-ground measurements. Very inexpensive.
Differential. A balanced input, non-isolated. Suitable for off-ground measurements provided that the sum of the common mode and normal mode voltages remains within the measurement range of the amplifier.
Single-ended, floating common. An isolated and quasi-balanced input (the floating common is typically connected to the (-) input of a differential amplifier). Suitable for off-ground measurements up to the breakdown voltage of the isolation barrier, and exhibits very good common mode rejection (100 db typical).
Differential, floating common. An, isolated and balanced input. Suitable for off-ground measurements to the breakdown voltage of the isolation barrier, and exhibits superb common mode rejection (>120 db).
For most industrial application that require isolation, the single-ended floating design provides the best price/performance.
