A high speed A/D converter based on the Resonant Tunneling Diode (RTD) is described. This A/D converter takes advantage of the folding characteristic of RTD to reduce the circuit complexity. The speed response of the A/D converter is improved by the fast latching action of the RTD digitizer. Simulations show that the 4-bit A/D converter can have a sampling rate greater than 16GHz. This performance represents a factor of 5 improvement over the best $-bit semiconductor A/D converter that has been reported.