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I have to design a 9U board working with 2 voltage supplies: +5 and +3.3. This card will be an upgrade design in an existing system. Due to limitations of the existing system the +5 supply comes already from the backplane connector while the +3.3 supply is a new one to the system and can only be connected from the front - on the side opposite the backplane.
As the logic on the card must work with the 2 supplies (eg - the logic between the SDRAMs and the output drivers) how should the grounds be designed in order to have the best performance? The board should operate at 40MHz.
I thank you for your help.
Thanks for your interest in High-Speed Digital Design.
One thing is clear: the grounds for the +5v and +3.3v regions must both be intimately connected to the digital logic ground plane on the card. Otherwise, the SDRAM section won't be able to send high-speed signals over to the output drivers.
This ground connection creates two tough problems.
First, your front-connected cables will include connections to both +3.3v and digital logic ground. The difficulty this creates is that any wires connected to the digital logic ground radiate terribly if exposed to the outside world.
The second difficulty has to do with whether the +3.3v supply will feed just one card, or multiple cards. Let's deal first with the radiation problem.
If you can't change the way the cable enters the card, here are a few ideas.
First: mock something up right away with your existing system and take it to the EMI range to see how it does. For this test, just solder a wire to the digital logic ground on the front of the card, and route it down to where you intend to mount the 3.3-V power supply. If this doesn't pass your emissions requirements, at least you'll learn how many dB of improvement are needed to fix the problem. On the other hand, if the card is inside a solid metal chassis with a good door, you might (just might) pass, in which case you don't need to do anything special.
If you need only a "few" dB of improvement you could install an EMI choke (common-mode choke) on the +3.3v power wiring. This choke is normally installed such that both the +3.3v supply and +3.3v return wires pass through the body of the choke. That way, the outgoing and returning currents cancel and so don't have to buy a choke large enough to handle the entire operating supply current without saturating. An improvement of 6dB is normally achievable with this sort of technique.
Here's a question related to the second issue: will the +3.3v supply be "floating"? To "float" the supply means that the output circuit is grounded only once, at the digital logic ground on the card where you use it. If the supply is floated then the +3.3v return wire carries only the 3.3v supply current. On the other hand, if you screw up and tie the +3.3v return to the chassis near the power supply, the difference in potential between the digital logic ground on your card (where one end of the +3.3v return wire is connected) and the chassis (where the other end of the +3.3v return wire is connected) may draw substantial amounts of DC current through the +3.3v return wire. This may require a heftier return wire, and will probably also saturate the common-mode choke you installed for EMI reasons.
The same thing happens if you attempt to power multiple cards from the same +3.3v power source. The differing DC voltages on the planes of the various card induce currents circulating among the +3.3v return connections. These currents will probably saturate your EMI choke.
Simple point: you want the +3.3v return connections to touch the cards near places where the cards are already well connected to the chassis, because at those points the DC potential differences are minimized. That's why so many people establish their power connections on the backplane side of the card.
Another simple point: routing the 3.3-V wire close to the chassis, or putting them in a metal conduit, can reduce radiation.
If all else fails, you may need to install a 5v to 3.3v DC-DC converter on each card.
It's possible that your design incorporates analog- to-digital converters, in which case the considerations become significantly more complex. If that's your problem, see these articles from my EDN "Signal Integrity" column: "ADC Grounding" and "Multiple ADC Grounding".
Dr. Howard Johnson