AD8331 VGA - Anyone use it?

Dec 30, 2007 1 Replies
R

All,



I am making an amplifier design (10 MHz max, 80 dB gain) using 2 stages of the AD8331

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on a 8-layer board design (for EMI). From the datasheet, it appears that they want you to decouple the LNA and the VGA supplies with ferrite beads on the power lines at the pins ...i.e. power plane --> ferrite --> pin



Now, the only place I can put these are on the top layer or the bottom. Top is a little filled up with the signal traces close to the pads. On the bottom, I risk a bunch of vias to send the power in at the pin.



I was wondering if anyone had used this IC and the ferrites and would share their expereince in placing those ferrites. I really want to put the ferrites on the top layer....but will have power and signal traces in close proximity. On the other hand, I have no idea what problems will be caused by sending it to the pad through vias to the ferrites on the bottom layer. Now, for my frequency range, this could be irrelevant.



But, if anyone has answers on this part or in general, that will be appreciated.



Thanks



Ray


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M

The industance of the VIAs might actually help in this reguard. The Ferrite bead is just a lossy inductor. Just make sure you have plenty of capacitance on the both sides and you should be fine.

Beware that low ESR caps can cause ringing so if your expected ripplecurrents are not high a medium ESR cap can be easier to play with.

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