DS90C032
Parameter Measurement Information
(Continued)
01194506
FIGURE 4. Receiver TRI-STATE Delay Waveforms
Typical Application
01194507
FIGURE 5. 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
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 termina-
tion 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 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.
TheDS90C032 differential line receiver is capable of detect-
ing 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 ef-
www.national.com
6
fects 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 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.
TheDS90C032 is a quad receiver
device, 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.
NSC [ NATIONAL SEMICONDUCTOR ]
