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MAX3292EPD Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX3292EPD Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 16 page Setting the Preemphasis Interval The MAX3291 has an internal fixed preemphasis interval of 100ns. Use the MAX3291 for existing designs requiring industry-standard ’75180 pin-compatibility at data rates of 5Mbps to 10Mbps. The MAX3292 has a resistor-programmable preemphasis interval for more flexibility. For data rates less than 1Mbps, use the following equation to calculate RPSET (the pre- emphasis setting resistor): RPSET = 580 (tBAUD - 100) where tBAUD = one baud period in ns. For example, a baud rate of 500kbps produces a baud period of 2µs (2µs = 2000ns). RPSET = 580 (tBAUD - 100) RPSET = 580 (2000 - 100) = 1.1MΩ For data rates of 1Mbps to 10Mbps, use the following equation to calculate RPSET: RPSET = 580 (tBAUD - 100)(tBAUD / 1000) where tBAUD = one baud period in ns. For example, a baud rate of 1Mbps produces a baud period of 1µs (1µs = 1000ns). RPSET = 580 (1000 - 100)(1000 / 1000) = 522kΩ (closest standard value is 523k Ω) Set the preemphasis interval by connecting the RPSET resistor from PSET to VCC. Use a 0.1µF bypass capaci- tor (CPSET) from PSET to VCC. If PSET is connected directly to VCC (RPSET = 0), the preemphasis interval reverts to the nominal 100ns value. Eye Diagrams One simple method to quickly determine your circuit configuration is to view an eye diagram. An eye dia- gram is a scope photo (voltage vs. time) showing the transitions of a pseudo-random bit string displaying at least one bit interval. Use an eye diagram to quickly calculate the total jitter of a circuit configuration. Jitter is the total time variation at the zero-volt differential cross- ing, and percent jitter is expressed as a percentage of one baud period, tBAUD. Figures 15 and 16 show typi- cal eye diagrams for a non-preemphasis device and the MAX3291/MAX3292. ISI and jitter are often used interchangeably; however, they are not exactly the same thing. ISI usually makes up the majority of the jit- ter, but asymmetrical high and low driver output voltage levels and time skews of non-ideal transceivers (driver and receiver) also contribute to jitter. RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication ______________________________________________________________________________________ 11 10,000 100 0.1 1 10 DATA RATE (Mbps) 1000 10% JITTER PREEMPHASIS 485 DRIVER LIMIT CONVENTIONAL 485 DRIVER LIMIT PREEMPHASIS REQUIRED FOR ERROR-FREE TRANSMISSION 24-GAUGE TWISTED PAIR Figure 14. Preemphasis Driver Performance Compared to a Conventional Driver Without Preemphasis at 10% Jitter Figure 15. Eye Diagram of a Typical RS-485 Transceiver Without Preemphasis, while Driving 1000 feet of Cable at 5Mbps Figure 16. Eye Diagram of the MAX3292 with a Preemphasis Interval of 175ns, while Driving 1000 feet of Cable at 5Mbps |
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