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Video amps bring it all into focus

Video amps have seen changes in requirements from customers in the last few years especially with the growth of LCD market in cell phones and televisions. Smaller packages with less power and better matching have become very important. Several companies, including Intersil, Linear Technology, Texas Instruments, and Fairchild Semiconductor offer amplifiers to meet the driver and buffer needs for video applications.

Designed to surpass the demands of high performance video applications, LT6553 from Linear Technology exceeds the resolution specification of 1600 x 1200 pixels,” according to applications manager Tim Regan. “The LT6553 is ideal for use in SXGA and UXGA LCD projectors and monitors, digital presenters, scanners, automotive display systems such as in-car navigation or in-car video systems, digital cameras and CCD imaging systems.”

The LT6553 incorporates an enable/disable function for easy multiplexing and signal routing. Each amplifier draws only 8mA and when disabled the supply current is reduced to 22uA. Furthermore, the amplifiers are capable of turning on in less than 50ns, making them ideal for spread spectrum and portable applications.


Linear Technology LT6553 amplifier for a video driver

“The newest members of Intersil's Elantec op amp family are designed to meet the requirements of high resolution display interfaces,” said Sameer Vuyyuru, Intersil's director of marketing for video and op amps. “The new EL536x family of triple current feedback amplifiers satisfies the demand for high bandwidth with low noise and, most importantly, controlled pulse response. These new devices' power consumption is in most cases less than any three individual amplifiers combined. The monolithic architecture also provides exceptional gain matching,” added Vuyyuru.


Intersil EL5360 Performance characteristics

“The Texas Instruments OPA693 eclipses previous video line driver solutions, providing an unsurpassed pulse response for emerging high resolution RGB applications. In addition to being the fastest +/-5 V capable fixed-gain amplifier on the market, the OPA693 offers considerably improved DC and AC precision at lower pricing than competitive devices,” said Michael Steffes, strategic marketing manager for TI's high-speed signal processing group. “Video applications will benefit from the exceptional pulse fidelity at high pixel rates offered by the 700 MHz bandwidth and 2500 V/us slew rate. The OPA693 is also the only fixed gain of +2 video line driver in the >250 MHz speed range that is capable of operating on a single supply from +5 V to +12 V.”


TI ultra wideband video buffer

Fairchild Semiconductor announces the FMS6418A, a highly integrated, triple video driver that is selectable for High Definition (HD) or Standard Definition (SD) signal filtering. The FMS6418A is used to remove high frequency noise prior to digitizing the signal (anti-aliasing), or to remove artifacts introduced during D/A conversion in the encoder (reconstruction filtering) for either RGB or YUV signals. This unique product offers improved performance, flexibility, and lower parts count for advanced video applications, including HDTV monitors, cable and satellite set top boxes, DVD players, Personal Video Recorders (PVRs), Video on Demand (VoD), and audio/video receivers. The FMS6418A has triple 6th order filters with selectable 30 MHz (HD) or 8 MHz (SD) frequencies. It also offers -38 dB of stopband rejection at 74.25 MHz for HD and -55 dB at 27 MHz for SD applications. The FMS6418A provides performance exceeding that of competitive integrated devices, while offering cost and design advantages over discrete solutions that require multiple components.


Fairchild Semiconductor triple video application

The FMS6418A combines a 2:1 multiplexer on each filter channel, with filtering and output drive amplification on all three channels, to provide a complete video solution. All channels provide 6 dB gain, accept 1V ground referenced inputs, and drive AC coupled loads.

What do applications need?
“One of the most demanding applications is in LCD projectors, where compact units with crisp, bright high resolution displays command a premium price,” said Erik Soule, Product Marketing Manager; Signal Conditioning Products. “For video amps, this translates into high bandwidth and slew rate, low power, small packages and good channel separation,” he said. A good example of this type of part is the company's new LT6553 which is a fixed gain of 2 RGB amp with over 650MHz GBW, 2500V/μs slew rate and 75dB channel separation at 10MHz.

“Another challenge is to drive video over long cables, for example taking advantage of legacy CAT5 cable,” continued Soule. “The new LT6552 is a video difference amp that solves this exact problem,” he added.


The LT6552 cleans up a video signal on 1,000 feet of twisted pair cable

“A video signal is only as good as the components in the chain,” observed Tushar Patel, Technical Marketing Engineer for National Semiconductor. “Video amp performance is key when designing video systems for any application. One such application is for wideband video routers and switchers with a bandwidth from 100MHz to 500MHz. To support such a bandwidth, an end-product would need internal op amps significantly higher in bandwidth – some say up to ten times the desired bandwidth of the box,” pointed out Mr. Patel.

“Additionally, keep in mind that there are ways to manipulate the bandwidth number advertised by manufacturers,” cautioned Patel. “The important BW number to look at is the large signal bandwidth (LSBW) specification, usually considered to be 1 volt peak-to-peak (Vpp) and higher. Many manufacturers advertise their small signal bandwidths (SSBW), however, generally speaking, video signals use large signals (2 Vpp). Customers may be enticed by the small signal specifications, however, products with the higher LSBW numbers have more value,” he concluded. .

National Semi recognized the need for LSBW and recently introduced the LMH6738, a triple wideband, 750 MHz operational amplifier, and the LMH6739 is a triple wideband, 750 MHz programmable gain buffer (PGB). Most importantly, with 400 MHz large signal bandwidth (LSBW), 3300 V/us slew rate, and a 0.1dB gain flatness of 200 MHz, the LMH6738/9 outperform the competition in driving high-resolution RGB (Red, Green, Blue) video signals. Additionally, the high output current of 90mA drives low impedance, high capacitive loads in single-ended cable line driving applications.

“Another important feature of video amps is the 0.1dB gain flatness,” said Alan Buchholz, Design Manager at National Semiconductor. “As a general rule, the higher gain flatness numbers support higher video resolutions. For example, HDTV systems need 0.1dB flatness out to about 30-50 MHz. On the other hand, when dealing with high-resolution video signals (computer graphics), you need a gain flatness out near 100-200MHz,” said Mr. Buchholz.

What should you look for when you are designing with video op amps? “Distortion becomes a key differentiator,” said Mr Buccholz. “It is important to minimize the amount of distortion the op-amp adds to the video signal. Most customers dealing with professional level equipment would consider second and third harmonic distortions around -80 to -90 dBc @ 5MHz a minimum requirement for their systems” observed Buccholz.

“Consequently, when dealing with composite video signals, differential gain/differential phase (DG/DP) numbers are important; typically values at or below 0.02%/0.02 are required for composite video applications,” added Mr. Buccholz. For example, in composite video, the color information is riding on top of the luminance information. Therefore the lowest DF/DP numbers will generate the cleanest and least distorted picture quality.

“For portable video like digitals still cameras (DSC) and cell phones with TV-output,” said Frank Haupt, Strategic Marketing Engineer at Texas Instruments. “The most important things are high integration, low power shutdown, small package size and easy connection to the embedded video D/A converter in the DSP chip. Good video specs obviously are important, but consumer video does not have the stringent requirements as broadcasting / professional video.”

“Voltage feedback is the paramount configuration for this application at the moment,” continued Mr. Haupt. The important features include:
– Integrated Filter to help reconstruct the DAC signal
– Low Power Shutdown. Best is the <5uA range
– low voltage, single supply operation 2.8-5V is best
– SAG correction helps to reduce AC coupling cap size and cost
– Capable of DC coupled output without clipping the sync pulse
– SC70, QFN or CSP packages

What can we expect?
“There are many trends that we see happening in the video industry, the important one is higher resolution,” said National's Tushar Patel. As video resolutions for portable products increase from VGA to SXGA and beyond, the op-amp speeds are going to have to increase as well. With this increase in resolution, op-amp requirements will have to improve their bandwidth (speed) and distortion performance, to keep up with video demands,” said Patel.

LTC's Erik Soule says it's the same as always, just more so. There will be a need for:
* Lower supply rails — meaning more single supply and rail to rail amplifiers.
* Smaller — meaning smaller packages, but also more features integrated on chip, such as gain resistors. Additionally, at higher bandwidths on-chip feedback paths improve signal integrity and reduces crosstalk.
* Lower power — a unique feature of Linear Technology's new LT6210/1 current feedback amplifiers is an “Rset” resistor, which allows the system designer to “dial-a-bandwidth,” essentially a speed-vs.-power knob. This means that the system designer can set up the amplifier so that it never draws more power than it needs for a given signal bandwidth.
* Higher speed — to accommodate higher screen resolutions such as SXGA, UXGA and the HDTV formats.

TI's Frank Haupt couldn't agree more stating that there will be higher integration, with filters, more functions, more channels, and eventually integration of the video driver into DSP chips. Additionally, there will be current output for lower voltage operation, DC coupled output operation because it helps eliminate big coupling capacitors (saving space and cost), smaller packages, and finally, a glueless interface to embedded video DACs.

Exciting designs are happening in video op amps. Now, you too, get the picture.


Company Information

Fairchild Semiconductor
Tel: 207-775-8100
www.fairchildsemi.com

Intersil Corp.
Tel: 321-724-7000
www.intersil.com

Linear Technology Corp.
Tel: 408-432-1900
www.linear.com

Maxim Integrated Products
Tel: 408-737-7600
www.maxim-ic.com

National Semiconductor Corp.
Tel: 800-272-9959
www.national.com

Texas Instruments Inc.
Tel: 800-477-8924, ext. 4500
www.ti.com

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