DS90C402_13 Datasheet, PDF (4/14 Pages) Texas Instruments – Dual Low Voltage Differential Signaling (LVDS) Receiver

DS90C402
SNLS001C – JUNE 1998 – REVISED APRIL 2013
TYPICAL APPLICATION
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Figure 3. Point-to-Point 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 3. This configuration provides a clean signaling environment for the quick 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 100Ω. A termination resistor of 100Ω 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 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 DS90C402 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. Both receiver 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 protection circuitry which will clamp the bus voltages.
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 receiver, 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 HIGH output voltage) for floating, terminated or shorted receiver inputs.
1. Open Input Pins. The DS90C402 is a dual receiver device, and if an application requires only one receiver,
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 ensure 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 100Ω 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 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
differential 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.
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