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I am studying a schematic which has a backplane card and another 6 cards interfaced on it. There is a power card which has EMI filters, transient protection module and an isolated DC DC regulator that taps from a 24V DC external source and provide 24V DC, 5V DC and 3.3V DC. There is another interface card that receives signals from a circular connector. This card processes the signals and sends it to an ADC circuit to be fed to another MPC card. I noticed that this card (Interface card) has its own DC DC regulators that tap 24V DC from an external source (similar to the power card) and provide 12V and 5V. My question is why can't the output from the power card be used to supply to the interface card? Why does the interface card has it's own regulators. I am guessing it's for isolation. Please explain.

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I think you've answered your own question. It's undoubtedly for isolation. The power card will probably have fairly large signals and higher currents which could cause fluctuations in their power supply even if regulated. If properly designed this would not be a problem.
The interface card may well be dealing with low level signals. Their signal processing circuits will need a quiet power supply.

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