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Given the 5870’s 188W board power (and even the Radeon HD 5850’s 151W rating), ATI had to pay particular attention to power consumption with this product. Voltages, clock rates, and on-board components were all taken into consideration.
To begin, there are the 5850-class frequencies, which facilitate lower voltages, and thus, lower board power (294W). The same power-saving technologies that allow other 5000-series cards to drop to remarkably-low idle consumption also apply here. Multi-level clock gating, voltage scaling, and engine/memory clock scaling all work effectively. In fact, we observed the same 157/300 MHz idle clocks seen on other cards from this same generation.
The Radeon HD 5970 adds a board-level feature that applies an ACPI S1 sleep state to the secondary GPU, effectively halving its idle power consumption, according to ATI. As a result, the Radeon HD 5970 is rated for a 42W idle board power—less than half that of a single-GPU Radeon HD 4870 (90W)!
In theory, then, there should be 15W separating a Radeon HD 5870 and 5970 at idle. Our measurements were a little higher at 27W between the two. Moreover, the Radeon HD 5970 actually seemed to use just a little more idle power than a pair of Radeon HD 5870s in CrossFire, suggesting the second GPU might not be dropping into S1 with our beta drivers.
So too is the 5970’s load consumption slightly higher than a pair of Radeon HD 5850s in CrossFire—though not far off the mark. Out of curiosity, we bumped up the voltages on our Radeon HD 5970 and pushed the clock rates to 5870 speeds (850/1,200 MHz). Seemingly not a fan of our FurMark test, the board quickly spiked to 99 degrees Celsius and then froze.
A single Radeon HD 5970 is slightly louder under load and slightly quieter at idle than two Radeon HD 5870s in CrossFire. The good news is that, on the Windows desktop, you’ll completely forget that there’s a foot-long dual-GPU monster under the hood. When it comes time to game, though, this card certainly makes its presence known.