2
\$\begingroup\$

I'm planning a mixed signal board with the following power rails:

  • +3.3V
  • +5V
  • -5V
  • +12V
  • -12V
  • +Adjustable voltage (2-10V)
  • -Adjustable voltage (2-10V)

Each of the rails are supplied from an external +24VDC supply via separate step-down DC/DC power modules (LT µModule silent switchers, LTM8074 and LTM8053).

The +3.3V rail is the only digital supply and powers the MCU and a few ICs. The other rails power three analog sections which consist mainly of op-amps for signal conditioning and driving/reading off-board transducers.

So far I have grouped and roughly placed components into three sections (+/-5V, +/-12V, +/-VADJ).

My question is regarding placement of the DC/DC power modules, where I have two options:

Option 1: Group them together close to the +24V input and route planes/polygons on the power layer to each section. This keeps switching noise away from the analog sections but might be more complex to route. Option 1

Option 2: Separate and place them close to the corresponding section and route/pour +24V across the board on a power layer to each power module.

Each section is more separated which allows for cleaner routing but might cause noise from the switchers to couple into the analog circuitry. Also requires the potentially noisy +24V rail to be routed across the board. Option 2

Which option would you prefer and why?

\$\endgroup\$
2
  • 1
    \$\begingroup\$ I depends entirely on your copper strategy on your PCB. \$\endgroup\$
    – Andy aka
    Feb 9 at 17:37
  • \$\begingroup\$ 6 layer stackup, SIG-GND-PWR-PWR-GND-SIG. I'm pretty confident I will be able to do all the signal routing on the two outer layers which allows for two dedicated power layers and two solid ground planes. \$\endgroup\$
    – Linder
    Feb 10 at 8:32

1 Answer 1

1
\$\begingroup\$

It's a tradeoff.

Option A limits distribution of the noisy 24V supply, and keeps the noisy converters away from sensitive analog circuitry. However, it requires routing your analog supplies through the noisy digital section. Option B keeps your analog supplies away from noise sources, but subjects your analog circuitry to the noise from supplies, and increases the distribution of the 24V supply, bringing it closer to the analog sections.

If you choose option A, the analog supply routing is the critical issue. Keep the +V and -V supplies routed differentially over a (or between two) unbroken ground plane(s). Carefully avoid noisy circuitry, since it has to run through the digital section.

If you choose option B, put the lion's share of the layout effort into keeping the current loops within the supply tight and as isolated as possible.

Personally, with no additional information about specific parameters or sources, I'd pick A.

\$\endgroup\$
2
  • \$\begingroup\$ For stackup I'm planning to do 6 layer: SIG-GND-PWR-PWR-GND-SIG. That way I can route/pour the positive and negative rails on separate layers sandwiched between two unbroken ground plane. Is that what you mean by routed differentially? \$\endgroup\$
    – Linder
    Feb 10 at 8:31
  • \$\begingroup\$ That's one way, and one of the more effective IMO. However, it will break up your 3.3V plane a bit. Keep the traces away from the 3.3V switcher, both to isolate them from noise and to preserve the integrity of the 3.3V plane beneath its own switcher. Also, keep the switching current loops on the outer layers with a single localized set of vias connecting it to the inner plane...no need injecting the switching noise to the inner layers. \$\endgroup\$ Feb 10 at 17:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.