I used ADM7151 for PLL+VCO power supply ,If the load 400mA current ，then the VCO output will uperimposed spur，if don't use ADM7151, the output is correct.，attachment are ADM7151 circuit diagrams and test phenomena ,so I want to know why ?
Hi fan XF,
It is unlikely that the LDO is the cause of the very high freq spurs that you are seeing, and it may be caused by some other elements in your setup. Sorting this out is very tricky, so to help isolate the cause, can you provide some more information in your setup? What's the power source to the LDO? How do you then connect the LDO to your application? And how do you measure the noise? Do you use an active load to draw the 400mA? Also, you said that the spurs went away when you did not use the ADM7151, can you explain this a bit more? What's the difference compared to the LDO powered setup? In parallel, I'll talk to some of our product experts to get more information.
Hi Jespirit ,thanks for ur reply ,you said power source ,it's a DC-DC output L/T's P/N to ADM7150 ,the customer not told us ,and we has calculate the junction temperature :25+1.5x0.42x36.9=48.2 (6.5V->5V,420mA),it's <<150°，so should ignored this ;another ,you said an active load to draw the 400mA ,we used amperemeters test current between ADM7151 and the back devices ;one more ,if DC-DC direct power supply to PLL,of course, the DC-DC output is adjusted to 5V ,the spurs will disappear
Apologies for the late reply, I had a discussion with the ADM7151 team and we're sure that the LDO itself does not contribute to the noise seen at this very high frequency. The LDO also do not have a switching element and could not cause any periodic noise. Possible culprit is the parasitics from the board, does the layout confirm with the recommended layout as stated in the ADM7151 datasheet? Another possible culprit is the DC-DC converter itself that's powering the LDO when it's output voltage is set to a higher level such as 6.5V. I urge the customer to try powering the LDO with a different DC power supply, perhaps a benchtop power supply and retake the measurements? It is also a good information to check the switching frequency of the DC-DC converter, and if that correlates with the spurs you are seeing.
Hi Jespirit ,I'm so sorry for the late reply.at present, the customer is in the test stage,and follow your advice ,any progress, I will reply to you，thanks！
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