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GeForce 7900 GX2: PCB Design

It’s not the first time an attempt to make a graphics card with two high-performance graphics processors on board is made. We could recall ASUS EN6800GT Dual, EN7800GT Dual or Gigabyte GV-3D1-68GT as examples.

And each time the developer was facing a lot of problems because it’s not just about putting two graphics chips on the PCB. Placing the memory chips, wiring the power circuit, and ensuring that the result can work at high frequencies are just some of the problems to be solved. It’s a terribly difficult job to design a PCB that would meet all the requirements, yet would have a normal size (installing an ASUS EN6800GT Dual into a standard system case was real troublesome).

All dual-chip graphics cards have been “single-storied” until the GeForce 7900 GX2. The developers have been sticking to traditional PCB designs although the concept of a two-storied graphics card isn’t new – it was first implemented back in 1998 in the Quantum 3D Obsidian2 X-24. That card was an ordinary Voodoo2 SLI made as a two-storied card, each storey accommodating one set of Voodoo2 chips.

Nvidia’s engineers took this path as they were developing their GeForce 7900 GX2. This graphics card is in fact two cards placed one above another.

The GeForce 7900 GX2 is also the longest graphics card for today. Its height is not bigger than that of existing graphics cards, so there should be no problems if you try to install it into narrow system cases, but its length means that not all PC cases can be used to build a Quad SLI system. In some system cases the cage for hard drives may get in the card’s way. On the other hand, the GeForce 7900 GX2 is going to ship in ready-made computers as yet, and you won’t have to solve that problem on your own.

Note that the left part of the top PCB is almost empty, but that’s not a waste of space, as you will see shortly. Seats for two more DVI-I connectors can be seen nearby. They are installed on the professional Quadro FX 4500 X2 graphics card that uses the same PCB design and can connect to four monitors simultaneously.

Despite the low power consumption of the G71 chip and the relatively low graphics memory frequency, the power circuit on the GeForce 7900 GX2 is rather complex and each PCB of this card is equipped with an independent voltage regulator. The MOSFETs in the bottom right corner of each PCB are cooled with a small aluminum heatsink. There’s a risk of overheat since the bottom heatsink is fully covered by the top PCB. That’s why the system case of a Quad SLI computer must be well ventilated. The PCI Express power connectors are rotated by 90 degrees relative to their usual position, probably to avoid making the card even longer. Each GeForce 7900 GX2 has two such connectors, so four external power connectors are attached to a whole Quad SLI subsystem that consists of two such cards.

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