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I am designing PCB2 & PCB3 but PCB1 & PCB4 are set in stone. My constraints are as follows:

1) The PMIC must be the 1st device to become alive. 2) As much as I would love to, I cannot use the 5V #1 supply for PCB4. PCB4 requires 2A of service but PCB1 can only supply 1A of current.

As you can see, U2 is supplied by 5V #1 and it's output signal is referencing 5V #1 as well, but, U3 is supplied by 5V #2 and it's input signal is (still) referencing 5V #1 (not 5V #2).

Note : All GNDs are common and the VIN (24V) originate from the same external 24V supply.

Can or will this (5V #1/5V #2) referencing mis-match cause excessive ground-loop/s, discontinuity and/or even function?

four interconnected PCBs

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  • \$\begingroup\$ Not clear what is the "referencing mismatch", Steve \$\endgroup\$ – analogsystemsrf Jun 17 '19 at 17:25
  • \$\begingroup\$ @analogsystemsrf What I meant by "referencing mis-match" is that U2 is 5V1 and U3 is 5V2. \$\endgroup\$ – Steve Jun 17 '19 at 17:31
  • \$\begingroup\$ Can you set maximum and minimum limits on the voltage of 5V1 (at board 3) and 5V2? If 5V1 drops by 500 mV between board 1 and board 3, it could certainly cause problems. If 5V2 could actually be 6 or 7 or 8 volts, it could certainly cause a problem. \$\endgroup\$ – The Photon Jun 17 '19 at 17:33
  • \$\begingroup\$ Make sure you put in plenty of decoupling on your supplies. Also try to use as many conductors as possible in your cables for ground. If you have any unused pins on your connectors, use them. \$\endgroup\$ – WhatRoughBeast Jun 17 '19 at 20:59
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No, this will not cause a problem. The high-level input voltage to U3 will be about 0.1V below the power supply voltage of U3. That input voltage is still well within the voltage range that will be recognized as a logic '1' by U3, and the slightly lower voltage will not damage U3.

The only possible disadvantage to this arrangement is that the power supply current drawn by U3 will be slightly higher than it would be if the high-level input came all of the way up to 5.2V. I doubt that you will even notice this if your board normally draws 2A.

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    \$\begingroup\$ It's unclear if 5V1 and 5V2 refer to 5.1V and 5.2V as is usually the case, or 5V supply one and 5V supply two. I'm guessing it's the latter. \$\endgroup\$ – John D Jun 17 '19 at 17:35
  • \$\begingroup\$ @ John D I was worried about that nomenclature. 5V1 and 5V2 are two different +5V supplies and not 5.1V and 5.2V. Sorry about that. \$\endgroup\$ – Steve Jun 17 '19 at 18:05
  • \$\begingroup\$ @ John D I updated the schematic;-) \$\endgroup\$ – Steve Jun 17 '19 at 18:16
  • \$\begingroup\$ @Steve As long as the two supplies are within 0.3V of each other then my answer still applies. \$\endgroup\$ – Elliot Alderson Jun 17 '19 at 19:30

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