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ADN4692E Datasheet(PDF) 8 Page - Analog Devices |
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ADN4692E Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page AN-1177 Application Note Rev. 0 | Page 8 of 12 JITTER, SKEW, DATA ENCODING, AND SYNCHRONIZATION With high speed differential signaling, such as LVDS and M-LVDS, accurate timing is critical to the performance of a system. PCB traces, connectors, and cabling can degrade the performance of data and clock signals, requiring that a margin for error is also present in system timing. This means that careful timing analysis may be required to achieve the maxi- mum throughput on an LVDS or M-LVDS communication link. Modern FPGAs and processors also have built-in capabilities to correct for timing errors, although there may be clearly defined limits to the amount of jitter tolerated, for example. WHAT IS JITTER? Jitter refers to the apparent movement of a signal edge with respect to the ideal time position of that signal edge. If a periodic signal is observed on an oscilloscope, the edges literally jitter back and forth with respect to the reference point. TIE EYE JITTER (PEAK- TO-PEAK) IDEAL ACTUAL (ONE PASS) ACTUAL (MULTIPLE PASSES) Figure 16. Waveforms Showing Time Interval Error, Jitter and Eye Jitter can be quantified simply as time interval error, the time difference between when a signal edge occurs, and when it should occur. Usually in order to determine the sources of jitter, a large number of TIE samples are recorded to build a histogram, from which deterministic jitter can be separated from random jitter. Total jitter can be quantified as a peak-to- peak value when bounded to a specific quantity of samples. The peak-to-peak value means the time difference between the earliest and latest edge observed during sampling. Peak-to-peak jitter can be seen visually if multiple waveform samples are overlaid on an oscilloscope display (infinite persistence), as shown in Figure 16. The width of the overlaid transitions is the peak-to-peak jitter, with the clear area in- between referred to as the eye. This eye is the area available for sampling by a receiver. Random jitter occurs due to noise, both electrical and thermal. The result is a Gaussian distribution to the time error, with this error introduced as random jitter. The jitter is unbounded; when more samples are recorded, the probability function continues to grow. Deterministic jitter is, by contrast, bounded. There is a fixed amount of this jitter in the system due to specific factors, such as the board layout and driver performance. Periodic jitter is one type of deterministic jitter and refers to the time difference between each cycle compared to the ideal. Periodic jitter is also recorded as a peak-to-peak value, that is, the difference between the longest and shortest periods observed WHAT IS SKEW? There are different definitions for skew, several of which are typically considered in designing high speed LVDS links. The most basic definition of skew is the difference in propagation time between the two signals in a differential pair. This means that edge transitions on one signal in a pair will not match up exactly with transitions on the complementary signal (the crossover will be asymmetric). INPUT ACTUAL OUTPUT IDEAL OUTPUT tPLH = tPHL tPLH tPHL PULSE SKEW ( tPHL – tPLH) D– D+ D– D+ D– D+ Figure 17. Waveforms Illustrating Pulse Skew Calculation Pulse skew on a differential signal refers to the difference between the low-to-high transition time (tPLH) and the high-to- low transition time (tPHL). This results in duty cycle distortion, that is, the bit period is longer or shorter for a Logic 1 or Logic 0. Pulse skew is illustrated in Figure 17. The blue waveform corresponds to an input signal, the green waveform to an ideal output (where propagation times on high-to-low and low-to- high transitions are matched), and the red waveform to an actual output, where the difference between tPLH and tPHL results in pulse skew. Channel-to-channel skew and part-to-part skew are some of the most important parameters in typical LVDS applications because they have multiple data channels that need to remain synchronized. Channel-to-channel skew refers to the difference, across all channels in a part, between the fastest and slowest low-to-high transition, or the fastest and slowest high-to-low transition (whichever is larger). Part-to-part skew extends this concept to channels across multiple parts. Skew across multiple channels (on one or multiple parts) is illustrated in Figure 18. The blue waveform corresponds to an input signal, with the four red waveforms comprising output channels on one or more parts. The difference between the fastest and slowest tPLH is calculated, along with the difference between the fastest and slowest tPHL. The channel-to-channel or |
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