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DS90LV028A Datasheet(PDF) 5 Page - National Semiconductor (TI) |
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DS90LV028A Datasheet(HTML) 5 Page - National Semiconductor (TI) |
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5 / 10 page  Applications Information (Continued) 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 DS90LV028A is a dual receiver device, and if an application requires only 1 receiver, the unused channel 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 power-off condition, the receiver output will again be in a HIGH state, even with the end of cable 100 Ω termination resistor across the input pins. The unplugged cable can become a floating antenna which can pick up noise. If the cable picks up more than 10mV of differential noise, the re- ceiver 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 ribbon cable. 3. Shorted Inputs. If a fault condition occurs that shorts the receiver inputs together, thus resulting in a 0V differ- ential input voltage, the receiver output will remain in a HIGH state. Shorted input fail-safe is not supported across the common-mode range of the device (GND to 2.4V). It is only supported with inputs shorted and no external common-mode voltage applied. External lower value pull up and pull down resistors (for a stronger bias) may be used to boost fail-safe in the presence of higher noise levels. The pull up and pull down resistors should be in the 5k Ω to 15kΩ range to minimize loading and waveform distortion to the driver. The common-mode bias point should be set to approximately 1.2V (less than 1.75V) to be compatible with the internal circuitry. Please refer to application note AN-1194 “Failsafe Biasing of LVDS Inter- faces” for more information. 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 reduction and signal quality. Balanced cables tend to gener- ate less EMI due to field canceling effects and also tend to pick up electromagnetic radiation a common-mode (not dif- ferential 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 (category 3) twisted pair cable works well, is readily available and relatively inexpensive. Pin Descriptions Pin No. Name Description 1, 4 R IN- Inverting receiver input pin 2, 3 R IN+ Non-inverting receiver input pin 6, 7 R OUT Receiver output pin 8V CC Power supply pin, +3.3V ± 0.3V 5 GND Ground pin Ordering Information Operating Package Type/ Order Number Temperature Number −40˚C to +85˚C SOP/M08A DS90LV028ATM −40˚C to +85˚C LLP/LDC08A DS90LV028ATLD www.national.com 5 |
Similar Part No. - DS90LV028A_05 |
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Similar Description - DS90LV028A_05 |
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