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AD8367 stability issue

Question asked by PeteW on Feb 5, 2013
Latest reply on May 3, 2013 by smith.

I'm trying to evaluate the AD8367 VGA and am running into problems with oscillations. I don't have the evaluation board but have mounted the device on a TSSOP adapter which is itself mounted on old RF eval board with a ground plane, so my ground leads, although not vias, are short. I'm trying to run it at 45MHz. I'm also using it in basic VGA mode, rather than AGC mode, and with a negative gain slope.  The trouble is, as I increase the gain the thing starts oscillating badly, with strong modes at ~200MHz. This happens when the gain voltage reaches ~0.35V. I did have an LC match to 50R on input and output, but have reverted to a straight resistive match to see if that stops the oscillations, but it hasn't.

 

My circuit is now as per Figure 33 in the datasheet, with a 68R shunt on the input (ac coupled) and a 10nF + 150R series on the output into a 50R load. Vp is +5V.

 

Any suggestions/recommendations would be appreciated.

Thanks,

Pete

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