How to connect LVDS differential clock to ALTMEMPHY pll_ref?

In Stratix III 340 device, I used alt_inbuf_diff. the inputs are two diffferential clock , output of alt_inbuf_diff is connected to ALTMEMPHY pll.

For Stratix IV 820 device, I tried in the same way, but i found that quartus ignores this alt_inbuf_diff. It seem that this buffer can only be used in StratixIII device.

How to do it in Stratix IV device?