Which is the earliest Apple TV that can mirror an iPad?
The Apple TV 2 is the earliest Apple TV that supports AirPlay Mirroring, being the earliest Apple TV capable of installing Software Update 4.4, which added the feature.


  AirPlay Mirroring: Wirelessly stream what's on your iPhone 4S or iPad 2 screen to your HDTV.


https://support.apple.com/kb/HT202157