Detail BAFO Display Port to HDMI Converter Cable Adapter (ACTIVE) BF-2651
DisplayPort to HDMI Adapter lets you connect an advanced digital monitor, such as the 20- or 23-inch Apple Cinema Display, to a Mac Pro (2009), Mac mini (2009), iMac (2009), MacBook, MacBook Pro, and MacBook Air with DisplayPort.
Active Adapters
Active adapters use additional chips to make the conversion inside the adapter, regardless of whether the source supports DP++. This means that active adapters are more expensive than passive adapters.
If you want to use multiple monitors with the same computer, you should use an active adapter because some video cards cannot run the maximum number of monitors while using DP++. This is especially true if the computer has more than one DisplayPort or Mini DisplayPort connection. Check with your video card manufacturer to confirm which type of adapter you need for the setup that you would like to run.