Many new homes and office buildings come prewired with more CAT5 wiring (Ethernet cable containing four unshielded twisted pairs) than present tenants need. Architects and planners have been asking: Why not use this infrastructure to distribute analog video?
After all, CAT5 is much less expensive than coax. Also, video digital-to-analog converters readily provide RGB or component video that can be sent over three of the CAT5's twisted pairs. Analog video driver and receiver chips tailored to the application have been needed, and they're now being supplied.
Keyboard-video-mouse (KVM) switch applications, which also use a twisted-pair transmission medium, have a similar need for analog video drivers and receivers (although KVM cabling is seldom as long as CAT5 building wiring).
Chipmakers have long offered triple and quad video amps for TVs, VCRs, and camcorders. Yet more of these makers are offering triple amps and receivers optimized for CAT5 and KVM products. It's not yet clear which application is the real driver behind this new thrust, but the trend is evident from a pair of new products from Analog Devices and Intersil.
The AD8143 triple differential receiver from Analog Devices distributes analog RGB signals representing resolutions up to 1600 by 1200 pixels via CAT5 cables. This device complements the company's AD8133 triple differential line driver, which offers 400-MHz, full-power, -3-dB bandwidth and a 1600-V/µs slew rate. It also reduces board space by up to 60% and cost by up to 45% compared to less integrated approaches.
The AD8143 exhibits a 70-dB common-mode rejection ratio (CMRR) at 10 MHz. Its ±10-V common-mode input range accommodates systems in which ground potential is different between the drive and receive ends. For KVM applications, it provides two auxiliary comparators in addition to the three differential receivers, which can be used to receive digital signals for keyboard or mouse functions from the fourth twisted pair of the CAT5 cable.
The ISL59830 triple video driver from Intersil targets CAT5 and KVM applications as well. It has a 300-MHz, -3-dB bandwidth. More importantly, it permits dc coupling on the input while drawing power from a single 3.3-V supply. While dc coupling normally eliminates the expense of coupling capacitors, it introduces the need for an additional, negative supply to ensure an input common-voltage range that accommodates the input signal with negative-going sync pulses.
By generating its own negative voltage internally, the ISL59830 gives designers the option of dc coupling without the need for an additional supply rail. (Alternatively, a VREF pin lets system designers level-shift the signal above or below ground to match an external reference.) The chip is hardwired for 6-dB gain on all three channels and can drive doubly terminated 75-W loads.
In addition to the ISL5983, Intersil offers chips that simplify equalization and prop-delay balancing in CAT5 applications.
In lead-frame chip-scale packaging, ADI's AD8143 receiver costs $2.55 and the companion AD8133 driver costs $2.59, both in 1000-unit quantities. Intersil's ISL59830 is $1.88, also in 1000-unit lots.