MD-82
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
Air Traffic Control Issued New Clearance; Flight Crew Returned To Departure Airport; Flight Crew Landed As Precaution; General Declared Emergency
Aircraft
I was the Pilot Monitoring.  On takeoff roll; Captain's airspeed indicator went to zero at approximately 100 knots.  First Officer's airspeed agreed with standby airspeed indicator.  Takeoff was continued.  ATC was advised that we were unable RNAV and were given a heading.  At this point the First Officer lost his airspeed as well with the standby airspeed consistent with phase of flight.  An emergency was declared and ATC was informed we were returning.  Shortly after our decision to return; the First Officer's airspeed came back and the QRH for CADC failure was accomplished.  CADC selector was placed to; 'Both on 2'; and indications returned to normal.  An overweight landing was accomplished.  Landing was at a normal descent rate. Weight at touchdown was approximately 135;500 pounds.  Safety responders check aircraft and brakes and the aircraft was taxied back to the gate.
MD-82 Captain experiences loss of his airspeed indication at 100 knots during takeoff and with the First Officer flying and the takeoff is continued. After becoming airborne the First Officers airspeed also fails and an emergency is declared. The First Officer's airspeed returns to normal shortly thereafter and the Captain's airspeed indicator is placed on number two CADC and the flight returns to the departure airport.