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DS90LT012AH Datasheet(PDF) 6 Page - Texas Instruments |
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DS90LT012AH Datasheet(HTML) 6 Page - Texas Instruments |
6 / 16 page NRND DS90LT012AH SNLS199A – SEPTEMBER 2005 – REVISED APRIL 2013 www.ti.com APPLICATION INFORMATION General application guidelines and hints for LVDS drivers and receivers may be found in the following application notes: LVDS Owner's Manual(SNLA187), AN-808(SNLA028), AN-977(SNLA166), AN-971(SNLA165), AN- 916(SNLA219), AN-805(SNOA233), AN-903(SNLA034). LVDS drivers and receivers are intended to be primarily used in an uncomplicated point-to-point configuration as is shown in Figure 5. This configuration provides a clean signaling environment for the fast edge rates of the drivers. The receiver is connected to the driver through a balanced media which may be a standard twisted pair cable, a parallel pair cable, or simply PCB traces. Typically the characteristic impedance of the media is in the range of 100 Ω. The internal termination resistor converts the driver output (current mode) into a voltage that is detected by the receiver. Other configurations are possible such as a multi-receiver configuration, but the effects of a mid-stream connector(s), cable stub(s), and other impedance discontinuities as well as ground shifting, noise margin limits, and total termination loading must be taken into account. The DS90LT012AH differential line receiver is capable of detecting signals as low as 100 mV, over a ±1V common-mode range centered around +1.2V. This is related to the driver offset voltage which is typically +1.2V. The driven signal is centered around this voltage and may shift ±1V around this center point. The ±1V shifting may be the result of a ground potential difference between the driver's ground reference and the receiver's ground reference, the common-mode effects of coupled noise, or a combination of the two. The AC parameters of both receiver input pins are optimized for a recommended operating input voltage range of 0V to +2.4V (measured from each pin to ground). The device will operate for receiver input voltages up to VDD, but exceeding VDD will turn on the ESD protection circuitry which will clamp the bus voltages. POWER DECOUPLING RECOMMENDATIONS Bypass capacitors must be used on power pins. Use high frequency ceramic (surface mount is recommended) 0.1 μF and 0.001μF capacitors in parallel at the power supply pin with the smallest value capacitor closest to the device supply pin. Additional scattered capacitors over the printed circuit board will improve decoupling. Multiple vias should be used to connect the decoupling capacitors to the power planes. A 10 μF (35V) or greater solid tantalum capacitor should be connected at the power entry point on the printed circuit board between the supply and ground. PC BOARD CONSIDERATIONS Use at least 4 PCB board layers (top to bottom): LVDS signals, ground, power, TTL signals. Isolate TTL signals from LVDS signals, otherwise the TTL signals may couple onto the LVDS lines. It is best to put TTL and LVDS signals on different layers which are isolated by a power/ground plane(s). Keep drivers and receivers as close to the (LVDS port side) connectors as possible. DIFFERENTIAL TRACES Use controlled impedance traces which match the differential impedance of your transmission medium (ie. cable) and termination resistor. Run the differential pair trace lines as close together as possible as soon as they leave the IC (stubs should be < 10mm long). This will help eliminate reflections and ensure noise is coupled as common-mode. In fact, we have seen that differential signals which are 1mm apart radiate far less noise than traces 3mm apart since magnetic field cancellation is much better with the closer traces. In addition, noise induced on the differential lines is much more likely to appear as common-mode which is rejected by the receiver. Match electrical lengths between traces to reduce skew. Skew between the signals of a pair means a phase difference between signals which destroys the magnetic field cancellation benefits of differential signals and EMI will result! (Note that the velocity of propagation, v = c/E r where c (the speed of light) = 0.2997mm/ps or 0.0118 in/ps). Do not rely solely on the autoroute function for differential traces. Carefully review dimensions to match differential impedance and provide isolation for the differential lines. Minimize the number of vias and other discontinuities on the line. Avoid 90° turns (these cause impedance discontinuities). Use arcs or 45° bevels. Within a pair of traces, the distance between the two traces should be minimized to maintain common-mode rejection of the receivers. On the printed circuit board, this distance should remain constant to avoid discontinuities in differential impedance. Minor violations at connection points are allowable. 6 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated Product Folder Links: DS90LT012AH |
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