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ADN4667 Arkusz danych(PDF) 5 Page - Analog Devices |
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ADN4667 Arkusz danych(HTML) 5 Page - Analog Devices |
5 / 12 page Application Note AN-1177 Rev. 0 | Page 5 of 12 DIFFERENTIAL SIGNALLING AND LVDS/M-LVDS Differential transmission is communication where two complementary signals are transmitted, with the received signal comprising the difference between the two signal lines. This form of communication, used by both LVDS and M-LVDS, has two distinct advantages, high noise immunity and low emissions. The high noise immunity arises because typically a noise source couples equally onto both signal lines, leaving the differential signal unaffected. Emissions from differential signaling are low due to the tight coupling between the two complementary signal lines when using a typical medium (twisted pair cable, or closely placed strip line). DEFINITIONS AND OUTPUT LEVELS For LVDS and M-LVDS, one signal line is noninverting (that is, high for a Logic 1 and low for a Logic 0) and the other signal line is inverting (that is, the complement of the noninverting signal). The difference in voltage between the two signal lines is termed the differential voltage, VOD. VOD is also shorthand for the magnitude of the differential voltage (positive or negative), or |VOD|. The two signal lines each have a maximum voltage swing of |VOD|, centered on the common-mode voltage, VOC (also referred to as the offset voltage, VOS). The differential voltage swings around 0 V. Typical LVDS signal levels are shown in Figure 8, together with the differential signal VOD and common-mode voltage VOC. In this figure, VOUT+ is the noninverting signal and VOUT− is the inverting signal. VOUT+ VOUT– 1.35V VOC = 1.2V 1.05V 0V LOGIC 0 LOGIC 1 LOGIC 1 0.3V –0.3V VOD (VOUT+ – VOUT–) |VOD| |VOD| Figure 8. LVDS Output Levels The differential voltage on an LVDS or M-LVDS bus is generated by a driver current source. Noninverting LVDS driver outputs or receiver inputs are generally denoted with a + and inverting driver outputs or receiver inputs with a −. Pin names are shown for the ADN4663 2-channel LVDS driver and ADN4664 2-channel LVDS receiver in Figure 9. M-LVDS follows the convention of RS-485 physical layer transceivers in naming the bus lines A for the noninverting signal and B for the inverting signal, or Y and Z for driver outputs on a full-duplex transceiver. DIN1 DOUT1+ DOUT1– VCC ADN4663 DIN2 DOUT2+ DOUT2– GND ROUT1 RIN1+ 100Ω 100Ω RIN1– VCC ADN4664 GND ROUT2 RIN2+ RIN2– Figure 9. ADN4663 and ADN4664 2-Channel LVDS Point-to-Point The distinction between LVDS and M-LVDS and other differential signaling standards is that they have a low output swing. The differential output voltage and common mode range specifications of LVDS and M-LVDS are shown in Figure 10. For LVDS, the output voltage swing, |VOD|, is a minimum of 250 mV and a maximum of 450 mV with a load of 100 Ω. This allows low power operation and ensures that while transitions are fast, to allow high data rates, the reduced output swing means that the slew rate is not too severe. Rise and fall times are generally in the region of hundreds of picoseconds, resulting in slew rates of around 0.5 V/ns to 2.5 V/ns. 250mV 450mV 480mV 650mV M-LVDS LVDS MIN VOD MIN VOD MAX VOD MAX VOD 0V TO 2.4V –1V TO 3.4V 4V 3V 2V 1V 0V –1V M-LVDS LVDS Figure 10. LVDS and M-LVDS Signaling Levels M-LVDS has slew-rate limited drivers to enhance the robustness of the signaling when there are additional impedance discontinuities from multiple drivers/receivers and stubs. This means that M-LVDS is limited to lower data rates compared to LVDS. The ADN4690E through ADN4697E are available with options for 100 Mbps or higher speed 200 Mbps. Another characteristic of M-LVDS is increased driver strength, resulting in a minimum output voltage swing |VOD| of 480 mV and a maximum of 650 mV with a load of 50 Ω (two termination resistors of 100 Ω, one either end of the bus). RECEIVER THRESHOLDS The receiver thresholds are the differential voltage levels above or below which the received signal is considered a Logic 1 or a Logic 0. For LVDS, a positive VOD >= +100 mV corresponds to a Logic 1 and a negative VOD <= -100 mV corresponds to a Logic 0. |
Podobny numer części - ADN4667 |
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Podobny opis - ADN4667 |
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