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Analog video signal requirements: Similarities and differences (Part 5)

Posted: 24 Aug 2007 ?? ?Print Version ?Bookmark and Share

Keywords:analog video? signal requirements? similarities and differences? Part 5?

By Randy Stephens
Texas Instruments

Analog video has been around for decades and is still in use today. The original and most common video standards include the National Television System Committee (NTSC) and Phase Alternation Line (PAL). Other modern consumer analog video transmission systems include S-Video, Component Video, professional G'B'R' video and computer R'G'B' systems.

This article will explore some of these analog video signal requirements and how they are similar, yet different, from each other and how to simplify the analog I/O design in these video systems.

Output capabilities
The most common output configuration of a line driver amplifier is to use AC-coupling. This is done to eliminate any possible DC-bias current flow and makes the system more universal. It is common to use between 220?F and 1000?F capacitors on the output of the amplifier to reduce line tilt.

In some systems, the DC-bias current is not a major concern. Instead, cost and PCB size may be more important. The THS73x3 allows DC-coupling without problems due to the rail-to-rail output amplifier stage. The output swing reaches within 100mV of the power supply and ground and can drive up to 80mA of current.

Other systems may require AC-coupling, but PCB size is also important. The flexibility of the THS7303 and THS7313 allows a method to achieve this goal〞SAG correction. SAG correction requires two capacitors, but these capacitors are much smaller〞nominally 47?F and 33?F and can achieve about the same tilt performance as a 330?F capacitor. Increasing these values to 68?F and 47?F respectively results in performance similar to a 470?F capacitor.

The SAG function obtains this result by utilizing an increase in gain as the frequency decreases. The amplifier gain counter-acts the 47?F capacitor drop resulting in an extension of the realizable pass-band. The 33?F capacitor is chosen to obtain a small peak, or Q-enhancement. This "cheats" the video system into achieving a respectable line-tilt or droop, especially with a field square wave signal.

Figure 10 shows the basic configuration of how SAG is implemented in the THS7303 and THS7313. SAG correction is also used by other manufacturers, but those systems often require much larger capacitors or larger power supplies to account for the larger DC gain that occurs in the system.

It is relatively easy to see how SAG functions. At DC, the gain increases due to the internal 675次 resistor added in series with the 878次 feedback resistor. At high frequencies where the output capacitor and SAG feedback capacitor are shorted out, the 675次 resistor is in parallel with the 150次 resistor, resulting in a 6dB system gain. This DC gain enhancement along with the proper ratio of capacitors allows the SAG function to simulate a much larger capacitor.

Figure 10

Figure 10: System level diagram of THS7303 showing the SAG feature
Click image to view Figure 10

Figure 11 shows the results of a 47?F + 33?F SAG connected output along with a 47?F, 100?F, and a 330?F traditional output configuration as shown at the Video Out point in Figure 10. This shows a small amount of peaking occurring that extends the performance of the system even further than if there were no peaking. Figure 12 shows the output voltage of the amplifier and the SAG feedback voltage.

Figure 11

Figure 11: Video output at receiver responses
Click image to view Figure 11

The THS7353 output is configured differently than the THS7303 and THS7313. The nominal gain of the THS7353 is 0dB, or unity gain. This is because the THS7353 was explicitly designed for an input system, such as for displays or DVD recorders. The front-end of these inputs are typically video decoders or video ADCs/scalers. As such, the allowable input range of these converters is typically less than 1.3Vpp. Hence the unity gain requirement. Additionally, the loading of an ADC is vastly different than a 150次 line. ADC front-ends are typically very high impedance, greater than 10k次, and typically have about 5pF to 10pF of capacitance. As such, the THS7353 was optimized for this loading which is a very different compensation than required for driving video lines.

Figure 12

Figure 12: SAG voltage responses and traditional response
Click image to view Figure 12

Also, the THS7353 can have gain adjustments configured externally. This allows the user to configure the gain to their needs. Sometimes it is a simple 0.5dB to 1dB flat gain. Other times it is desirable to counteract a SinX/X characteristic that may exist with a DAC. In other systems a long cable may be connected which has skin-effect losses that need to be compensated as shown in Figure 13.

Figure 13

Figure 13: THS7353 driving an ADC with cable equalization
Click image to view Figure 13

In some systems, the DAC output voltage capability requires higher gains than the 6dB gain offered by the THS7303 or THS7313. There are two simple ways to solve the problem: use the THS7353 and an external gain resistor, or use the THS7303 / THS7313 SAG feedback point. If an external resistor is placed between the SAG feedback point and ground, the gain of the amplifier becomes:

Formula

Figure 14 shows this configuration.

Figure 14

Figure 14: THS7303/THS7313 with higher gain configuration
Click image to view Figure 14

For example, adding an external 726次 resistor between SAG and ground results in a system gain of 4V/V, or 12dB. The drawback to this feature is the DC output level also increases and care must be taken to ensure there are no clipping issues. Additionally, the output amplifier is a voltage feedback amplifier (VFB) which has a gain-bandwidth (GBW) product limitation. While this will not seriously affect the filter characteristics, it will affect the THS7303 filter bypass bandwidth inversely with gain. For example, with a gain of 4V/V, the bandwidth of the THS7303 in bypass mode becomes approximately 90MHz. The THS7353 also has a GBW limitation with a default unity gain. Increasing the gain beyond 2 or 3V/V will seriously start reducing bandwidth. Additionally, the compensation of the THS7303 was designed explicitly for driving the video line and as such, is best suited for DAC buffering.

Control and interface
To allow these features and flexibility to exist in the THS73x3 products, I?C control is employed. GPIO was considered, but there would need to be at least 8 to 10 pins to control all the options on a reasonable manner. SPI was also considered due to its higher immunity to noise and timing over I?C. But, because almost every encoder, decoder, video processor, etc. already uses I?C, it makes perfect sense to use the same system already there. To ensure there are no I2C address conflicts, there are 2 pins that allow up to 4 addresses to be set on the THS73x3.

Conclusions
The flexibility of the filters, input biasing, input MUX, output configuration, I?C control, single supply with operation from 2.7V to 5V, and very low power consumption is unmatched in the industry. Couple all of these features with a very reasonable price and the THS73x3 represents a major breakthrough in video filter amplifiers that should work in just about every video system.

For Part 1, please click here.

For Part 2, please click here.

For Part 3, please click here.

For Part 4, please click here.

About the author
Randy Stephens
is a member group technical staff at Texas Instruments Inc.




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