Now we use AD725 to change VGA signal to PAL signal.I have a question：can AD725 support maximum resolution of 1600×1200@60hz，if it can not support，what is the maximum resolution of VGA signal input？
No, it can not support 1600x1200@60
The AD725 is intended for NTSC and PAL signals. The limit is set by the low pass filters.
640x480 VGA however this has to output in an interlaced format which most current video cards will not do.