TIA/EIA-644-A drivers must generate a VOD of 247 mV to 454 mV across 100 Ω, while 899 drivers require more voltage, 480 mV to 650 mV, across the smaller load of 50 Ω. The reasons for the difference fall back to the extra noise contributors in the multipoint environment versus point-to-point discussed earlier. Beyond the basic function, the symmetry of the complementary outputs becomes an important characteristic for noise emissions from the line. Ideally the current in each conductor of the signal pair is equal and opposite; as usual, the ideal is too expensive to achieve. This means that there is always some inequality in the two currents called imbalance. Imbalance can result from differences in the electrical characteristics of the complementary driver output circuitry or the loads, or from time delay between switching outputs. This is particularly critical, as the common-mode components created during switching imbalance are high in frequency and radiate readily. The differential driver parameter of significance in this regard is the common-mode output voltage, VOC (sometimes designated a VOS), and is the arithmetic mean of the two complementary output voltages. You measure this driver characteristic directly by monitoring the output voltage at the node between two identical resistors connected across the line as shown in Figure 2–2. The common-mode output can be broken down to the steady-state and peak-to-peak parameters VOC(SS) and VOC(PP) with emitted noise frequency fundamentals of 0.5/tUI Hz with VOC(SS) and out to 3/tUI Hz and beyond with VOC(PP).
The smaller the VOC(PP) parameters are, the better the noise emission performance of the interface. Both TIA/EIA-644 and -899 require their drivers to create no more than 150 mV of VOC(PP). When comparing the radiation potential of a data line, VOC(PP) is directly comparable to the voltage swing of a single-ended signal, and the advantage of differential over single-ended signaling in this regard becomes obvious.
| Title | LVDS Driver Output Voltage |
| Description | The driver’s primary function is to launch an electromagnetic wave into one or two transmission lines. As explained earlier, this is equival... |
| Review Rating | 4/5
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