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DS90C032B Datasheet(PDF) 5 Page - National Semiconductor (TI) |
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DS90C032B Datasheet(HTML) 5 Page - National Semiconductor (TI) |
5 / 9 page Typical Application Applications Information 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 en- vironment for the quick edge rates of the drivers. The re- ceiver 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 im- pedance of the media is in the range of 100 Ω. A termination resistor of 100 Ω should be selected to match the media, and is located as close to the receiver input pins as possible. The termination resistor converts the current sourced by the driver into a voltage that is detected by the receiver. Other configurations are possible such as a multi-receiver configu- ration, 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 DS90C032B differential line receiver is capable of de- tecting signals as low as 100 mV, over a ±1V common-mode range centered around +1.2V. This is related to the driver off- set voltage which is typically +1.2V. The driven signal is cen- tered around this voltage and may shift ±1V around this cen- ter 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 ef- fects of coupled noise, or a combination of the two. Both re- ceiver input pins should honor their specified operating input voltage range of 0V to +2.4V (measured from each pin to ground), exceeding these limits may turn on the ESD protec- tion circuitry which will clamp the bus voltages. Receiver Fail-Safe: 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 or terminated receiver inputs. 1. Open Input Pins. The DS90C032B 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 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 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 intercon- nect should be used. Twisted pair cable will offer better balance than flat ribbon cable. The footprint of the DS90C032B is the same as the industry standard 26LS32 Quad Differential (RS-422) Receiver. For additional LVDS application information, please refer to National’s LVDS Owner’s Manual available through Nation- al’s website www.national.com/appinfo/lvds. DS100990-7 FIGURE 5. Point-to-Point Application www.national.com 5 |
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