  We have developed an FX-architecture digital spectro-correlator for the
Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeter
Array. The correlator is able to simultaneously process four pairs of dual
polarization signals with the bandwidth of 2 GHz, which are received by up to
sixteen antennas. It can calculate auto- and cross-correlation spectra
including cross-polarization in all combinations of all the antennas, and
output correlation spectra with flexible spectral configuration such as
multiple frequency ranges and multiple frequency resolutions. Its spectral
dynamic range is estimated to be higher than 10^4 relative to Tsys from
processing results of thermal noise for eight hours with a typical correlator
configuration. The sensitivity loss is also confirmed to be 0.9 % with the same
configuration. In this paper, we report the detailed design of the correlator
and the verification results of the developed hardware.
