iPad - erase data completely

Solution 1:

Data on an iOS device [3GS & later] is protected by hardware encryption.

Wiping the device using Apple's guide at What to do before selling or giving away your iPhone, iPad, or iPod touch is quite sufficient.

Extract :

• If you paired an Apple Watch with your iPhone, unpair your Apple Watch.
• Back up your iOS device.
• Tap Settings > iCloud. Scroll down and tap Sign Out. In iOS 7 or earlier, tap Delete Account.
• Tap Sign Out again, then tap Delete from My iPhone and enter your password.
• Go back to Settings and tap General > Reset > Erase All Content and Settings. * If you turned on Find My iPhone, you might need to enter your Apple ID and password.
• If asked for your device passcode or Restrictions passcode, enter it. Then tap Erase [device].
• Contact your carrier for help transferring service to a new owner. If you aren't using a SIM card with your device, you can contact them to get help transferring service to the new owner.

See also : Apple KB - iOS: Understanding data protection
On recent iOS the setting is under Touch ID & Passcode.