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Abracadabra — Maxim Admits to Using a Little Black Magic to Create Highly Integrated Video Filters and Buffers


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Maxim MAX7454 Diagram”

Sunnyvale, Calif. — Maxim Integrated Products Inc. introduces the MAX7454 and MAX7455 3-channel, switchable high definition (HD)/standard definition (SD) video-reconstruction filters and buffers for 1080i, 720p, 480p, and 480i video scanning-system standards.

Each channel of the MAX7454 and MAX7455 combines a lowpass filter with a video buffer capable of driving a standard 150Ω video load with a 2-V peak-to-peak signal. The magnitude response of the lowpass filter can be switched between a 10-MHz and 30-MHz bandwidth through a control pin, which allows optimal filtering for HD, standard definition interlaced (SDi), or standard definition progressive scan/noninterlaced (SDp) encoder digital-to-analog converter (D/A converter) outputs. The input and output signals are DC-coupled to the MAX7454 and MAX7455, eliminating AC-coupling caps that consume board space and add cost. These devices are ideal for anti-aliasing or D/A converter-output video-reconstruction applications in digital video devices such as HD STBss, as well as HDTVs as CRTs, LCDs, plasmas, and DLPs.

The MAX7454 has an output buffer gain of +6 dB and the MAX7455 has an output buffer gain of +12 dB. The 3-channel devices are ideal for RGB or Y Pr Pb component video signals in interlaced, progressive scan, or high-definition formats. In HD mode (fc = 30 MHz), each filter channel achieves -55 dB of attenuation at 74.25 MHz, with -2.3 dB of attenuation at 30 MHz. In SD mode (fc = 10 MHz), each filter channel achieves -62 dB of attenuation at 27 MHz, with -0.40 dB of attenuation at 10 MHz. A fine-adjustment control pin is available to optimize the corner frequency of the filter response by 15%.

The MAX7454/MAX7455 save significant board space and design time, while providing increased performance at a cost comparable to the discrete high-definition L-R-C filter and buffer solutions typically used today. The MAX7454/MAX7455 are offered in a 20-TSSOP package and operate over the upper commercial temperature range (-0 degrees Celsius to +85 degrees Celsius).

Prices start at $2.75 (1000-up, FOB USA).

Click here for a preliminary data sheet.

Readers can call 1-800-998-8800.

Think of the MAX7454 and MAX7455 triple switchable HD/SD video reconstruction filters and buffers as the aspirin designers can avoid by using Maxim's devices, instead of designing their own filters and buffers using some 40 to 50 different discrete components.

Filter designs can be very tricky, said Ken Fields, a member of Maxim's technical staff. “I don't want to say it's black magic, but it borders on that if you don't have the experience to do this,” he said. Making these filters work properly can be a problem because of all the parasitics that exist on the board, which comes from taking a bunch of discrete capacitors, resistors and inductors and putting them into a small space — making it difficult to get good performance, Fields said.

Maxim has taken all of these discrete components, which are subject to noise and component variations, and replaced them with a single device that is simpler to use than an op amp, Fields said. “You don't need to set the gain or anything. Just plop it on the board and use it.”

Furthermore, Maxim's device can be used for both high definition (HD) and standard definition (SD) video signals, Fields said. While Maxim is no stranger to video filters, this is the company's first filter aimed at the HDTV market, he said.

These new products are extensions of a very large family of filters that Maxim has had for a number of years, Fields said. Typically, the market for this type of device has been dominated by the older standard definition video format. “These are our first devices aimed at the new emerging high definition standard,” Fields said. “High definition TV has been in the public consciousness for a while but it's only recently that people are starting to buy them,” he added.

Designers can easily switch one pin on the filter and it changes from the original standard definition to the high definition standard. That makes these devices backward compatible, Fields said. Some designers are using two totally different filters — one for the old standard and one for HDTV and switching between them.

Highly integrated filters weren't available until recently. Historically, discrete components have been used to achieve the same functionality. There are some integrated parts out there but they don't offer very good performance, Fields said.

Another advantage of using this part is that it allows the designer to tweak or optimize performance with a fine-adjustment control pin. If the designer wants better passband, he adjusts the setting to one position. If the stopband performance is more important, he sets it to a different position. It gives the engineer the freedom and flexibility to get further optimization in the system, Fields said.

The architecture Maxim chose minimizes all external components and specifically eliminates AC-coupling capacitors. Most filter and buffer designs use AC coupling, which results in performance degradation and line time distortion. “We figured out a way to match the DC levels out of the DAC or encoder perfectly with the MAX7454 and MAX7455 for a completely DC coupled circuit eliminating these performance limitations,” Fields said.

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