Design Article
Video current D/A converters: some fundamentals for IP use
John Kusching, vice president of engineering, Qualcore Logic, Inc.
10/5/2005 12:09 PM EDT
Digital to analog converters are used in video systems to convert digital video data and sync information into analog signals. These analog signals directly drive monitors or TV’s, usually via a cable. A common example would be a personal computer, where the graphics chip inside the PC would contain the Video D/As which in turn, directly drive the video cable going to the monitor. For reasons of cost, Video D/A converters are usually fabricated using a CMOS process. This also allows for easy integration of the Video D/As into a larger CMOS graphics chip.
Typical Video D/A converters have 8 to 10 bits of accuracy and operate at speeds of between 100 to 400 MHz. At these speeds, current D/As are the preferred architecture since current D/As are less sensitive to capacitive loads and can also easily swing to ground. Current D/As will also directly drive a video cable, eliminating the need for a buffer between the DAC outputs and the cable. Video D/As typically have three channels, although two and four channels D/As are also used. The three channel D/As are usually referred to as RGB DACs, signifying the Red, Green and Blue signals which are transmitted to video monitors (See figure 1).
Figure 1: Video D/A converter architecture
Since the DACs utilize a current output architecture, a method is needed to convert the output currents into voltages. This is done using resistors, with one set of resistors placed on the graphics card near the graphics chip outputs in the PC and another set of resistors placed inside the monitor. Usual values for these resistors are 75 ohms and when used with a video cable with a characteristic impedance of 75 ohms, creates a balanced transmission line system with proper terminations that will minimize reflections. Using this system, the resistive load that each DAC drives is 37.5 ohms and using a full scale swing of 1 volt gives a full scale current output requirement of about 26.7mA. Another popular full scale voltage swing is 0.7V, giving a full scale current requirement of 18.7mA.
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