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DS90LV047ATM Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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DS90LV047ATM Datasheet(HTML) 6 Page - National Semiconductor (TI) |
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6 / 12 page  Applications Information (Continued) The TRI-STATE function allows the driver outputs to be dis- abled, thus obtaining an even lower power state when the transmission of data is not required. The DS90LV047A has a flow-through pinout that allows for easy PCB layout. The LVDS signals on one side of the de- vice easily allows for matching electrical lengths of the differ- ential pair trace lines between the driver and the receiver as well as allowing the trace lines to be close together to couple noise as common-mode. Noise isolation is achieved with the LVDS signals on one side of the device and the TTL signals on the other side. Power Decoupling Recommendations: Bypass capacitors must be used on power pins. Use high frequency ceramic (surface mount is recommended) 0.1µF 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 10µF (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 layers (top to bottom); LVDS signals, ground, power, TTL signals. Isolate TTL signals from LVDS signals, otherwise the TTL 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 differen- tial 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 re- flections 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 re- jected by the receiver. Match electrical lengths between traces to reduce skew. Skew between the signals of a pair means a phase differ- ence between signals which destroys the magnetic field can- cellation benefits of differential signals and EMI will result. (Note the velocity of propagation, v = c/Er 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. Care- fully review dimensions to match differential impedance and provide isolation for the differential lines. Minimize the num- ber or 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 allow- able. Termination: Use a termination resistor which best matches the differen- tial impedance or your transmission line. The resistor should be between 90 Ω and 130Ω. Remember that the current mode outputs need the termination resistor to generate the differential voltage. LVDS will not work without resistor termi- nation. Typically, connecting a single resistor across the pair at the receiver end will suffice. Surface mount 1% to 2% resistors are best. PCB stubs, component lead, and the distance from the termination to the receiver inputs should be minimized. The distance between the termination resistor and the receiver should be < 10mm (12mm MAX). Probing LVDS Transmission Lines: Always use high impedance (> 100k Ω), low capacitance (< 2 pF) scope probes with a wide bandwidth (1 GHz) scope. Improper probing will give deceiving results. Cables and Connectors, General Comments: When choosing cable and connectors for LVDS it is impor- tant to remember: Use controlled impedance media. The cables and connec- tors you use should have a matched differential impedance of about 100 Ω. They should not introduce major impedance discontinuities. Balanced cables (e.g. twisted pair) are usually better than unbalanced cables (ribbon cable, simple coax.) for noise re- duction and signal quality. Balanced cables tend to generate less EMI due to field canceling effects and also tend to pick up electromagnetic radiation a common-mode (not differen- tial mode) noise which is rejected by the receiver. For cable distances < 0.5M, most cables can be made to work effectively. For distances 0.5M ≤ d ≤ 10M, CAT 3 (cat- egory 3) twisted pair cable works well, is readily available and relatively inexpensive. Fail-Safe Feature: The LVDS receiver is a high gain, high speed device that amplifies a small differential signal (20mV) to CMOS logic levels. Due to the high gain and tight threshold of the re- ceiver, care should be taken to prevent noise from appearing as a valid signal. The receiver’s internal fail-safe circuitry is designed to source/sink a small amount of current, providing fail-safe protection (a stable known state of HIGH output voltage) for floating, terminated or shorted receiver inputs. 1. Open Input Pins. The DS90LV048A is a quad receiver device, and if an application requires only 1, 2 or 3 re- ceivers, the unused channel(s) inputs should be left OPEN. Do not tie unused receiver inputs to ground or any other voltages. The input is biased by internal high value pull up and pull down resistors to set the output to a HIGH state. This internal circuitry will guarantee a HIGH, stable output state for open inputs. 2. Terminated Input. If the driver is disconnected (cable unplugged), or if the driver is in a TRI-STATE or power- off condition, the receiver output will again be in a HIGH state, even with the end of cable 100 Ω termination resis- tor across the input pins. The unplugged cable can be- come a floating antenna which can pick up noise. If the cable picks up more than 10mV of differential noise, the receiver may see the noise as a valid signal and switch. To insure that any noise is seen as common-mode and not differential, a balanced interconnect should be used. Twisted pair cable will offer better balance than flat rib- bon cable. www.national.com 6 |
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