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LVDS problem on the De2-70

Altera_Forum
Honored Contributor II
‎12-18-2009 06:46 AM
1,194 Views

I use de2-70 board to test the LVDS voltage values. 

My design is posted below. 

Now I got a odd problem. 

The signal derectly outputed by PLL, its average voltage is 1.2V and maximum voltage is 2.5V. But the signal outputed by the counter, its average voltage is 1.2V and maximum voltage is 4.4V and its also have the negative voltage value. 

I think it's so weird. Is my design right?  

P.S. the vccio is 2.5V.
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Altera_Forum
Honored Contributor II
‎12-18-2009 02:39 PM
534 Views

Seems like you never consulted the Cyclone II LVDS specification. Unlike Cyclone III or high end FPGA, Cyclone II doesn't provide a true LVDS driver. It needs external resistors, either in the 3R (Device Handbook Figure 11-3) or the 1R/RSDS configuration (Figure 11-7). In addition, a 100 ohm differential load at the receiver must be present to get the correct LVDS level. 

 

The DE2-70 board isn't actually wired for differential signaling. You have additional 47 ohm series resistors in the input protection network that must be taken into consideration.
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