DS90LT012AQ Datasheet, PDF (5/8 Pages) National Semiconductor (TI) – Automotive LVDS Differential Line Receiver

−100mV to 0V. This is useful for fail-safe biasing. The thresh-
old region is shown in the Voltage Transfer Curve (VTC) in
Figure 4. The typical DS90LT012AQ LVDS receiver switches
at about −30mV. Note that with V_ID = 0V, the output will be in
a HIGH state. With an external fail-safe bias of +25mV ap-
plied, the typical differential noise margin is now the difference
from the switch point to the bias point. In the example below,
this would be 55mV of Differential Noise Margin (+25mV −
(−30mV)). With the enhanced threshold region of −100mV to
0V, this small external fail-safe biasing of +25mV (with respect
to 0V) gives a DNM of a comfortable 55mV. With the standard
threshold region of ±100mV, the external fail-safe biasing
would need to be +25mV with respect to +100mV or +125mV,
giving a DNM of 155mV which is stronger fail-safe biasing
than is necessary for the DS90LT012AQ. If more DNM is re-
quired, then a stronger fail-safe bias point can be set by
changing resistor values.
30063929
FIGURE 4. VTC of the DS90LT012AQ LVDS Receiver
FAIL SAFE BIASING
External pull up and pull down resistors may be used to pro-
vide enough of an offset to enable an input failsafe under
open-circuit conditions. This configuration ties the positive
LVDS input pin to VDD thru a pull up resistor and the negative
LVDS input pin is tied to GND by a pull down resistor. The pull
up and pull down resistors should be in the 5kΩ to 15kΩ range
to minimize loading and waveform distortion to the driver. The
common-mode bias point ideally should be set to approxi-
mately 1.2V (less than 1.75V) to be compatible with the
internal circuitry. Please refer to application note AN-1194,
“Failsafe Biasing of LVDS Interfaces” for more information.
PROBING LVDS TRANSMISSION LINES
Always use high impedance (> 100kΩ), 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 important
to remember:
Use controlled impedance media. The cables and connectors
you use should have a matched differential impedance of
about 100Ω. They should not introduce major impedance dis-
continuities.
Balanced cables (e.g. twisted pair) are usually better than
unbalanced cables (ribbon cable, simple coax) for noise re-
duction and signal quality. Balanced cables tend to generate
less EMI due to field canceling effects and also tend to pick
up electromagnetic radiation a common-mode (not differential
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 rela-
tively inexpensive.
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