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Big Oscillation seen at HMC799LP3E Transimpedance Amp

Question asked by gt_engineer on Oct 12, 2015
Latest reply on Jun 30, 2017 by salemdar

Hello,

 

We have designed several small PCBs around the HMC799 without issues. However on the latest new design we have, we observed a very noticeable sinusoidal oscillation of about 860MHz and 520mV of amplitude on the output of HMC799 (see attached picture). This happened WITHOUT any input signal and only has the 5V power supply. There is also no other active components on the board. The things we have tried and the observation are listed below:

 

1. Ground the input of HMC799, the strong output oscillation (OSC) disappears

2. cut the signal trace connecting to the input down to about 20mils, the OSC amplitude reduced by about 400mV but the frequency did not change.

3.  When I removed the 22nF capacitor connected to pin4 (CEXT), the OSC disappears.

4. Suspecting of an EMI/EMC issue, we took the system down to a EM chamber and the oscillation is still there.

 

We have followed the reference schematic design provided in the HMC799 datasheet. From our debugging steps above, would you please comment on where else could this oscillation come from? What steps to take to prove and disprove?

 

Thanks a lot!

 

David

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