Isolation filter for VCCA in Cyclone-IV E

[Altera_Forum]

Dear Sir, 

 I want to know the type of isolation filter circuit for connecting 2.5V to VCCA(Analog power to PLL) in Cyclone IV FPGA. I am referring Figure 2. Example Cyclone IV E power supplies block diagram in Cyclone IV Device family Pin Connection Guidelines.  Where do I get reference designs based on Cyclone-IV FPGA.  Please help.  Best Regards, Thulasi

[Altera_Forum]

Higher impedance ferrite beads (e.g. 300 to 600 ohm) in combination with the VCCA bypass capacitors are usually fine.

[Altera_Forum]

Hi FvM, 

 Thanks a lot your reply. But, any altera document which mentions the above ?   Best Regards, Thulasi

[Altera_Forum]

You'll find different suggestions in different Altera documents. Since ever, either ferrite beads or low value resistors have been suggested for filtering of sensitive pins, e.g. PLL analog supplies. On the other side, you'll notice that many development kits are tying VCCA to the general vcc25 IO voltage without additional filtering. You can read this in a way that the IO voltage is achieving sufficient low ripple (e.g. < 0.3 V as required in the CV pin connection document) without additional filtering, which sounds plausible. 

 I use to keep a 600 ohm/1A ferrite bead for VCCA. This might be a reminiscence to Cyclone and Cyclone II FPGAs that didn't have internal PLL voltage regulators and were rather susceptible to supply voltage ripple, resulting in PLL loss of lock. Cyclone III and above are hand-tame compared to it.