Typical Application
Applications Information
LVDSdrivers 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 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 DS90C032 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 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 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.
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 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 of HIGH output voltage) for
floating, terminated or shorted receiver inputs.
1. Open Input Pins. The DS90C032 is a quad receiver de-
vice, and if an application requires only 1, 2 or 3 receivers, 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 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.
The footprint of the DS90C032 is the same as the industry
standard 26LS32 Quad Differential (RS-422) Receiver.
DS011945-7
FIGURE 5. Point-to-Point Application
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