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Hello I want to make PCB for ADF5901 24GHz power amplifier, it says use 3 capacitor for each supply pin, 10pF, 1nF and 100nF. we have about 6 supply pin. Can we remove 10pf bypass from all pins, If using 4 layer board with 2 parallel power plane? How we can become sure that this is correct decision? Answer that I've found from guys advise and answer, the engineers world is EV-boards and manufacturers recommendation, specially if you have no test equipment and time.

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    \$\begingroup\$ Deviate at your peril. You've been given a recipe that works for at least one layout. Are you prepared with test equipment, time, and experience to find a simpler arrangement? \$\endgroup\$
    – glen_geek
    Dec 30, 2018 at 19:36
  • \$\begingroup\$ Allow me to reverse the question: why do you think you can remove those capacitors? \$\endgroup\$
    – user39382
    Dec 30, 2018 at 19:51
  • \$\begingroup\$ It is better to have it and not use it than to need it and not have it. You have a 24GHz part. I'd implement the eval board layout. \$\endgroup\$ Dec 30, 2018 at 19:51
  • \$\begingroup\$ @duskwuff Because of capacitance between two power plane. \$\endgroup\$ Dec 30, 2018 at 19:52
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    \$\begingroup\$ @dmb It's learning project, but I have not time & test equipment, I tried to get ride of it because the IC is small and I can't place the small caps right near the IC and the board is messy. now I can't delete because someone answered, ty so much. \$\endgroup\$ Dec 30, 2018 at 20:45

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Answer: ADF5901 = 3 sets of 3 capacitors.

From ADF5901 data sheet:

[Pin No] 4, 5 [Mnemonic] TX_AHI [Description] Voltage Supply for the Tx Section. Connect decoupling capacitors (0.1 μF, 1 nF, and 10 pF) to the ground plane as close as possible to this pin. TX_AHI must be the same value as AHI.

The datasheet repeats this 4 separate times.

But the eval board shows three sets of capacitors adjacent to the ADF5901 for TX_AHI, AHI and VCO_AHI. RF_AHI and DVDD appear to be connected to AHI.

ADF5901

And three sets of capacitors adjacent to the ADF4159 for AVDD, DVDD and VP. These pins do not go to the ADF5901.

ADF4159

The designers recommend these capacitors. It is better to have them and not use them rather than need them and not have them. I'd follow layout used on eval board. You know it works.

There is a second evaluation board and the same capacitor distribution is used.

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  • \$\begingroup\$ Thanks for great explanation, you mean referring to EV-board, 3 set will be sufficient for ADF5901 and 6 set is optional for ADF5901 referred to datasheet, isn't that? \$\endgroup\$ Dec 30, 2018 at 20:32
  • \$\begingroup\$ Looks like 3 sets for ADF5901 and 3 for ADF4159. I'd look at eval board PCB layout. \$\endgroup\$ Dec 30, 2018 at 20:50
  • \$\begingroup\$ Ok, but only for ADF5901 referred to datasheet we have 6 set optional and 3 set needed?? \$\endgroup\$ Dec 30, 2018 at 21:02
  • \$\begingroup\$ I've tweaked my answer to clarify the 6 sets you talk about. For the ADF5901, you need 3 sets. \$\endgroup\$ Dec 31, 2018 at 3:21
  • \$\begingroup\$ I'm confused in it's datasheet they've told you must connect 3 cap to each supply pin, isn't that? And it have 6 power pin. For example for TX_AHI in datasheet: "Voltage Supply for the Tx Section. Connect decoupling capacitors (0.1 μF, 1 nF, and 10 pF) to the ground plane as close as possible to this pin. TX_AHI must be the same value as AHI.", we need 2 set of 3 cap for these two pin since it didn't use: "these", instead used: "to this pin", means one set for each, isn't that? But I think it will be super nice and totally 3 set is enough as you said referring to EVs. \$\endgroup\$ Jan 1, 2019 at 8:12

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