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How We Measure Power Consumption

Tom's Hardware Graphics Charts: Performance In 2014
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Measurement Equipment and Methodology

Our power consumption test setup was planned in cooperation with HAMEG (Rohde & Schwarz) to yield accurate measurements at small sampling intervals, and we've improved the gear continuously over the past few months.

AMD’s PowerTune and Nvidia’s GPU Boost technologies introduce significant changes to loading, requiring professional measurement and testing technology if you want accurate results. With this in mind, we're complementing our regular numbers with a series of benchmarks using an extraordinarily short range of 100 μs, with a 1 μs sampling rate.

We get this accuracy from a 500 MHz digital storage oscilloscope (HAMEG HMO 3054), while measuring currents and voltages with the convenience of a remote control.

The measurements are captured by three high-resolution current probes (HAMEG HZ050), not only through a riser card for the 3.3 and 12 V rails (which was custom-built to fit our needs, supports PCIe 3.0, and offers short signal paths), but also directly from specially-modified auxiliary power cables.

Voltages are measured from a power supply with a single +12 V rail. We're using a 2 ms resolution for the standard readings, which is granular enough to reflect changes from PowerTune and GPU Boost. Because this yields so much raw data, though, we keep the range limited to two minutes per chart.

Measurement Procedure
Contact-free DC measurement at PCIe slot (using a riser card)
Contact-free DC measurement at external auxiliary power supply cable
Voltage measurement at power supply
Measurement Equipment
1 x HAMEG HMO 3054, 500 MHz digital multi-channel oscilloscope
3 x HAMEG HZO50 current probes (1 mA - 30 A, 100 kHz, DC)
4 x HAMEG HZ355 (10:1 probes, 500 MHz)
1 x HAMEG HMC 8012 digital multimeter with storage function
Power Supply
Corsair AX860i with modified outputs (taps)

A Lot Can Happen in 100 Milliseconds...

...and we mean a lot! Let’s take a look at an analysis of all three voltage rails using a 2 ms sample across 100 ms (giving us 50 data points). Just looking at those results makes us pity the power supply. Jumps between 91 and 355 W over the auxiliary power connectors are pretty harsh. The fluctuations aren't as crazy on the other rails.

On the bright side, neither AMD nor Nvidia graphics cards with auxiliary power connectors fully utilize the 75 W made available through a PCI Express x16 slot. That hasn't always been the case. Additionally, there's far less variance over the PCI Express interface, no doubt benefiting stability.

High-Resolution Measurements

We wrap this part of our introduction up with illustrations of power consumption at idle and under a gaming workload. Again, all of this will get explained in more detail in an upcoming article.

Here's what's interesting: AMD's Radeon R9 290X demonstrates an idle power figure under 14 W. However, the many peaks up to 32 W skew that figure up if you're sampling more slowly. With on-board memory factored out, really, all that's left of the power use comes in under 12 W.

The differences aren't just apparent at idle, either. Power consumption under the effects of a gaming workload also turns out to be lower than what older/slower equipment would have us believe. Those massive disparities between our gear and slower equipment only showed up in the last two generations of AMD's hardware, so it's a fairly recent phenomenon. But it does mean the company gets beaten up more than it should in most reviews.

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