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Under pressure

Video amplifiers are being put under pressure by the latest high definition consumer requirements. By Brian Black.

Video op amps, like the good engineers who design with them, are constantly being asked to do more with less. Higher video resolution requires higher analogue video signal bandwidths – and that means ever faster amplifiers. At the same time, designers are looking for ways to achieve these higher resolutions with lower voltages and even single supply rails. Fortunately, a new crop of signal conditioning products can achieve this.
For some time now, amplifiers have been able to achieve standard definition video and svga (800 x 600pixel) using low voltage supplies. For example, the LT6550 and LT6551 offer 110MHz bandwidth and 400V/µs slew rate, yet can operate on supplies as low as 3.3V. Until recently, however, high resolution video such as uxga (1600 x 1200pixel) required amplifiers with supplies of 6V or higher. This wider supply range is needed to avoid clipping the signal, because the fastest op amps typically require additional headroom.

Video signal characteristics
To better understand the increasing performance requirements of video amplifiers, it’s helpful to take a closer look at the video signals involved.
Digital studio equipment for NTSC broadcast television typically uses pixel rates of around 14million per second. In contrast, the xga computer format (1024 x 768) operates at around 80Mpixel/s. The latest high definition consumer formats put out a comparable 75Mpixel stream and the increasingly popular uxga professional graphics format (1600 x 1200) generates a whopping 200Mpixel/s.
As a result, achieving accurate video reproduction with these newer formats is placing exceptional demands on the frequency response of video amplifiers. Specifically, pulse amplitude waveforms like those of baseband video generally require reproduction of high frequency content up to at least the fifth harmonic of the fundamental frequency component. This is 2.5 times the video pixel rate. So, for uxga resolution, a frequency response up to and beyond 500MHz is required. Whilst products such as the LT6553 – with 650MHz bandwidth, 1700V/µs slew rate and 6ns settling time – can achieve uxga speeds and resolutions, they typically require more than a single 5V supply to do so because of the video signal amplitude.

Brian Black

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