DS90LV048A_16 Datasheet, PDF (14/22 Pages) Texas Instruments – 3-V LVDS Quad CMOS Differential Line Receiver

DS90LV048A
SNLS045C – JULY 1999 – REVISED JULY 2016
www.ti.com
Layout Guidelines (continued)
11.1.2 Differential Traces
Use controlled impedance traces that match the differential impedance of your transmission medium (that is,
cable) and termination resistor. Run the differential pair trace lines as close together as possible as soon as they
leave the IC (stubs must be < 10 mm long). This helps eliminate reflections and ensure noise is coupled as
common-mode. In fact, we have seen that differential signals which are 1 mm apart radiate far less noise than
traces 3 mm apart because magnetic field cancellation is much better with the closer traces. In addition, noise
induced on the differential lines is much more likely to appear as common-mode which is rejected by the
receiver.
Match electrical lengths between traces to reduce skew. Skew between the signals of a pair means a phase
difference between signals, which destroys the magnetic field cancellation benefits of differential signals and
EMI, results. Remember the velocity of propagation, v = c/Er where c (the speed of light) = 0.2997 mm/ps or
0.0118 in/ps.
Do not rely solely on the autoroute function for differential traces. Carefully review dimensions to match
differential impedance and provide isolation for the differential lines. Minimize the number or vias and other
discontinuities on the line.
Avoid 90° turns (these cause impedance discontinuities). Use arcs or 45° bevels.
Within a pair of traces, the distance between the two traces should be minimized to maintain common-mode
rejection of the receivers. On the printed-circuit board, this distance must remain constant to avoid discontinuities
in differential impedance. Minor violations at connection points are allowable.
11.1.3 Termination
Use a termination resistor that best matches the differential impedance or your transmission line. The resistor
must be between 90 Ω and 130 Ω. Remember that the current mode outputs need the termination resistor to
generate the differential voltage. LVDS does not work without resistor termination. Typically, connecting a single
resistor across the pair at the receiver end will suffice.
Surface mount 1% to 2% resistors are best. PCB stubs, component lead, and the distance from the termination
to the receiver inputs must be minimized. The distance between the termination resistor and the receiver must be
< 10 mm (12 mm maximum).
11.2 Layout Example
LVCMOS
Inputs
Decoupling Cap
DS90LV047A
DS90LV048A
1 EN
DOUT1- 16
1 R_IN1-
EN 16
2 D_IN1
DOUT1+ 15
2 R_IN1+
ROUT1 15
3 DIN2
DOUT2+ 14
3 R_IN2+
ROUT2 14
4 V_CC
5 GND
DOUT2- 13
DOUT3- 12
4 RIN2-
5 R_IN3-
VCC 13
GND 12
6 D_IN3
DOUT3+ 11
6 RIN3+
ROUT3 11
7 D_IN4
DOUT4+ 10
7 R_IN4+
ROUT4 10
8 EN*
DOUT4-
9
8 R_IN4-
EN* 9
Input Termination
(Required)
Figure 22. Layout Recommendation
Series Termination (optional)
Decoupling Cap
LVCMOS
Outputs
Series Termination (optional)
14
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