Files with atdf extension can be found as special data analysis files in ASCII variant of Standard Test Data Format.
ATDF file extension - ASCII Test Data Format
What is atdf file? How to open atdf files?
File type specification:
File extension atdf is used for ASCII variant of STDF format. The difference is that the original .stdf files are binary and thus faster to analyze, compared to .atdf files that are text based thus can be read using any text editor.