try to convert them using ACIS format then Import in to autocad
while exporting the oject in to acisi format make sure that the faces selected not the Features
If you try Raju's method, you probably will want to set your ACIS output options to Version 7.0, as I believe that was the last common version between ACIS and AutoCAD.
I have had many more problems with exporting ACIS than I have with STEP or IGES. You might have to go with one of the latter. STEP is usually better than IGES, but the receiving system may force you to use whatever they are capable of receiving.