What does the CT (center tap) value mean in a transformer datasheet?
It means what it says. A transformer winding that is center-tapped has an extra connection (the center-tap) in the center of the winding. 

Sounds circular, doesn't it. 

This allows several options when using the winding. You can get half the voltage, or do impedance matching (you get a 4:1 ratio), or treat the center-tap as AC ground and get half-voltage signals that are 180 degrees out of phase. 







simulate this circuit &ndash; Schematic created using CircuitLab