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PCB Design and Cooling System

Sapphire HD 4850 X2 2G/1G GDDR3

Like the Radeon HD 4870 X2, the Sapphire HD 4850 X2 represents a highly sophisticated design. Having the same dimensions as the GeForce GTX 280, it has to carry two GPUs with two sets of memory chips, a PCI Express switch chip, and an appropriate power circuit. However, ATI Technologies’ closest partner was not taken aback and developed a unique PCB for this product. It has nothing to do with ATI’s reference design. The card is an imposing view with the cooler’s casing installed:

The two fans seem to promise a low level of noise, but, running a little ahead, we should confess that the card is very noisy due to its weak GPU heatsinks. Besides, the four DVI-I connectors left no space on the card’s mounting bracket for vent holes that could be used to exhaust the hot air out of the system case.

As a result, the hot air from the card remains within the system. The RV770 core being of a hot temper, you have to take care about proper ventilation of your system case. The cooler’s casing can be removed easily. You can unfasten four screws near each fan and have a look at this:

So, each GPU is cooled with an individual heatsink/fan pair. The heatsinks are made from aluminum but have a copper sole in the place of contact with the GPU die. They are rather small and light, so each heatsink is fastened with four spring-loaded screws only, without back-plates. The common dark-gray thermal grease is used as a thermal interface here.

You may wonder why the manufacturer didn’t use a more efficient single heatsink that would cool both the GPUs simultaneously. Well, it just wouldn’t be easy to ensure proper contact between such a heatsink and the components. Moreover, the row of electrolytic capacitors located between the heatsinks above the PCI Express switch would get in the way then. The switch itself is cooled with an individual needle-shaped heatsink fastened with thermal glue. We didn’t dare to tear it off, but it must conceal a PLX Technology PEX8647 chip, the same chip as on the Radeon HD 4870 X2.

Interestingly, all the memory chips of the Sapphire HD 4850 X2 are placed on the face side of the PCB whereas the reference Radeon HD 4870 X2 has half of the chips on the reverse side of the PCB. The chips are cooled with simple L-shaped aluminum plates fastened with three spring-loaded plastic clips. The thick elastic pads ensure proper thermal contact. This should be enough to cool GDDR3 memory considering the airflow from the fans.

The Sapphire HD 4850 X2 carries a total of 16 memory chips (QimondaHYB18H1G321AF-10) with a capacity of 1Gb (32Mb x 32), voltage of 1.8V and rated frequency of 1000 (2000) MHz. 16 such chips make up two memory banks with a capacity of 1024MB and a 256-bit memory bus: one bank for each GPU. The two banks store two copies of the same data due to the specifics of CrossFire technology, so the total amount of local memory available to 3D applications is 1024 megabytes. The memory is clocked at 993 (1986) MHz, which corresponds to the official Radeon HD 4850 X2 specs. There is little room for overclocking since the memory chips have an access time of 1 nanosecond.

The GPUs were both manufactured on the 37th week of 2008, i.e. in early September. Their memory controllers are reprogrammed for the use of GDDR3 instead of GDDR5 but you can’t distinguish the two versions of the RV770 chip by its marking. The marking offers no information for the uninitiated, save for the manufacturing date. The Sapphire HD 4850 X2 processors are clocked at 625MHz, which corresponds to the official specs, too. Considering the small heatsinks and the fact that the Radeon HD 4850 has a peak power draw of 110 watts, we can’t expect the Sapphire HD 4850 X2 to be quiet or economical.

The power circuit has nothing to do with that of the Radeon HD 4870 X2. Highly sophisticated, it is mostly located on the reverse side of the PCB, with only a few components nestled between the two GPUs. There is a two-phase voltage regulator based on a uPI uP6201BQ controller with three Infineon 042N03LS power transformers in each phase. The purpose of this regulator is rather unclear. It may be responsible for the memory chips and the PCI Express switch but can also take part in regulating the GPU voltage. The larger portion of the power circuit is hidden under two aluminum heatsinks: it is a seven-phase regulator with two external power connectors, one of which is of the 8-pin 150W variety, just like on the Radeon HD 4870 X2. The use of this connector is justifiable as the card is going to have a peak consumption of 220-230W. The connectors are placed near each other, which makes it easier to connect power cables to them than to the power plugs of the Radeon HD 4870 X2. Anyway, the connection process would be even handier if the connectors were turned by 90 degrees as on the GeForce GTX 280.

As for the four DVI connectors, this graphics card is indeed capable of supporting four monitors at any resolutions up to 2560x1600. As opposed to Nvidia’s SLI, ATI’s CrossFireX technology does not impose any limit on the number of monitors: this number depends only on many GPUs and display controllers the card has. The reference designs of the X2 series cards developed by ATI provide for the installation of only two DVI ports, and only one of them is used usually. Sometimes a large panel is connected to the second port via a DVI → HDMI adapter. Thus, it is unclear what user category is targeted by Sapphire with this solution. The Sapphire HD 4850 X2 is not a professional graphics card and won’t be used for appropriate applications. The support for four monitors simultaneously might be considered as an advantage if its implementation did not worsen the cooling system: the card lacks vent slits in its mounting bracket and cannot exhaust the hot air out.

Besides the DVI ports, the card is equipped with a standard 7-pin mini-DIN connector that can output video in Composite, S-Video and YPbPr formats. It also has one CrossFire connector for building a four-GPU subsystem out of two Sapphire HD 4850 X2 cards. But this would be a dangerous experiment considering that the cards do not exhaust the hot air out of the system case. It is no good having two heaters with a combined dissipated power of 440-460W inside your computer.

Thus, the Sapphire HD 4850 X2 seems to have a good design, save for two points. We don’t grasp the purpose of the developer’s idea of installing four DVI ports, and the cooling system is obviously ineffective for such an advanced card.

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