Solution:

Referrnig to the secion C.7 of Appendix C in the PCI-1716's manual,We can find the A/D Status Register"GATE". This register"GATE" is set to disable in Advantech DLL. So when we excute the DLL example and use the external trigger, we can ignore the signal "TRG_GATE" and only connect our clock source to the PIN "EXT_TRG". Setting the mode to external Clock source in the AP.then we can use the external clock.