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A New Power Consumption Test Setup

Nvidia GeForce GTX 970 And 980 Review: Maximum Maxwell
By , Igor Wallossek

Nvidia's newest architecture presents us with a whole new set of challenges for measuring power consumption. If the maximum of all four possible rails are to be measured exactly (to find out Maxwell’s power consumption reduction secrets), then a total of eight analog oscilloscope channels are needed. This is because voltage and current need to be recorded concurrently at each rail in real-time. If the voltages are measured and then used later, the result may be inaccurate. So, how did we solve this problem?

We enlisted the help of HAMEG (Rohde & Schwarz) to search for a solution with us. In the end, we had to use two oscilloscopes in parallel (a master-slave triggered setup), allowing us to accurately measure and record a total of eight voltages or currents at the same time with a temporal resolution down to the microsecond.

The measurement intervals need to be adjusted depending on the application in question, of course, in order to avoid drowning in massive amounts of data. For instance, when we generate the one-minute graphs for graphics card power consumption with a temporal resolution of 1 ms, we have the oscilloscope average the microsecond measurements for us first.

We use a riser card on the PCIe slot (PEG) to measure power consumption directly on the motherboard for the 3.3 and 12 V rails. The riser card was built specifically for this purpose.

In addition, we separately measure the voltage and current at each of the two individual PCIe power connectors.

Test MethodologyNo-contact current measurement at all rails
Direct voltage measurement
IR real-time monitoring
Test Equipment2 x HAMEG HMO3054, 500 MHz Four-Channel Oscilloscope with Data Logger
4 x HAMEG HZO50 Current Probe
4 x HAMEG HZ355 (10:1 Probe, 500 MHz)
1 x HAMEG HMC8012 DSO with Data Logger
1 x Optris PI450 80 Hz Infrared Camera + PI Connect
Test SystemIntel Core i7-5960X, 4.2 GHz
16 GB G.Skill Ripjaws DDR4-2666 (4 x 4 GB)
MSI X99 Gaming 7
2 x Transcend SSD370 (System, Applications + Data, Storage)
be quiet! Dark Power Pro 1200 W
Microcool Banchetto 101

Nvidia’s GPU Boost Accelerates Maxwell

Everything makes sense in theory, but we still want to know how Maxwell achieves better efficiency at this magnitude. Kepler already adjusted the GPU’s voltage quickly and exactly depending on its load and temperature, and AMD’s PowerTune did the same thing as well. It turns out that Maxwell refines the formula further. With its shaders fully utilized, the new architecture's advantage over Kepler practically vanishes. So, Maxwell depends on its superior ability to adjust to changing loads, and, consequently, it’s able to tailor the power consumption even better to the needs of the application in question. The more variance there is, the better Maxwell fares.

To illustrate, let’s take a look at how Maxwell behaves in the space of just 1 ms. Its power consumption jumps up and down repeatedly within this time frame, hitting a minimum of 100 W and a maximum of 290 W. Even though the average power consumption is only 176 W, the GPU draws almost 300 W when it's necessary. Above that, the GPU slows down.

Now, how do the PSU’s important 12 V rail's current and voltage behave under these conditions? We add them up for this purpose.

The PSU doesn’t supply a constant 12 V supply voltage. Spikes that empty the secondary capacitors and the slower power supply that tries to fill them back up again cause voltage fluctuations that the graphics card's five phases must take care of. All of these interactions make our measurements more complicated than a log of averages would suggest.

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  • 0 Hide
    Ce3in , 19 September 2014 13:24
    Excellent Review!

    Looks like The Gtx 980 will become a part of my build family!
  • 0 Hide
    HEXiT , 19 September 2014 20:31
    went to buy 1 form ocuk today as they had around 70 in stock of both the 970 and 980... but by the time i sorted the order (about 10 mis) they were out of stock...
    so roll on next week and hopefully they will have them back in stock...
  • 1 Hide
    Marco Washa , 20 September 2014 06:09
    These cards are really impressive. GG nVidia you have done right this time.
    -Top performance as always
    -Low power consumption
    -A right price this time XD
  • 2 Hide
    Marco Washa , 20 September 2014 06:12
    These cards are really impressive. GG nVidia you have done right this time.
    -Top performance as always
    -Low power consumption
    -A right price this time XD
  • 0 Hide
    shaunwil , 20 September 2014 09:47
    Bought a 980 from scan. By 2pm they had sold almost 300 cards in the day. Looking forward to 4k G-sync with the card now when the monitor comes available.
  • 0 Hide
    Omar101 , 21 September 2014 22:51

    nvidia
  • 0 Hide
    Alpha3031 , 22 September 2014 05:00
    Now, if AMD actually made some progress with power. That should drive prices down too.
  • 0 Hide
    Mahisse , 22 September 2014 11:42
    I fear this may be the beginning of the end for AMD GPU. Seems like Nvidia is beating AMD on any entry level now with a better cost/performance ratio. I want competition not monopoly!
  • 0 Hide
    Alpha3031 , 23 September 2014 03:15
    Triple post?
  • 0 Hide
    Mahisse , 23 September 2014 09:30
    Don't know how that happened but they are deleted now :) 
  • 0 Hide
    Bitty , 9 October 2014 14:51
    Just getting back into gaming after a huge gap. Got a Gigabyte G1 gtx970 (another later) to replace a very well-behaved AMD 9790 GHz. All seemed ok until I hit replay in Grid2 when it froze. Repeatedly it did this with oc or not but temps etc were fine. hmmmmm. Valley worked like a charm with none and big overclocks - no artifacts. Titanfall would freeze too. Asetto Corsa was fine. I looked about and saw a few complaining of similar issues thinking the 344.16 drivers were the cause. I suspec ted that power might be an issue since other factors were okay and not everybody has the issue. I then found this article on power draw and the penny dropped.

    I have an oldish Antec TP New Series 650 watt modular psu. I had the gtx970 on one 25A 12v rail feeding the 8+6 connectors. It seems this was possibly not enough to cope with the peaks. Not up on psus but with 80% efficiency that 25x12 is not a full 300 watts - more like 240watts. So, I added another 12v rail for the 6pin and hey presto it worked fine no issue.

    So anyone with such an issue might try this or get a more powerful psu.
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