1108 View Datasheet(PDF) - Fairchild Semiconductor

Part Name

Description

Manufacturer

1108

LVDS 8 Port High Speed Repeater

Fairchild Semiconductor 

Applications

Signal Optimization via Internal Termination

For LVDS signaling in point-to-point applications, receivers

or repeaters with on-chip termination are preferable to

reduce the overshoot or undershoot due to the reflection

caused by stubs at receiver inputs. As a rule of thumb, usu-

ally the termination resistor for an LVDS receiver should be

placed as close as possible to the receiver, especially for

high speed applications. If the distance between termina-

tion resistors and receivers is too long, the interconnection

will be seen as an un-terminated stub which can produce

reflections resulting in higher EMI. Internal termination can

effectively smooth out this ringing which can otherwise

jeopardize the receiver noise margin. This is important for

reliable high-speed operation with tighter required signal

settling times. Below is a list of the advantages/disadvan-

tages of internal termination.

Internal termination is not suitable for all applications. In

order to set a proper VOD at the driver outputs, receivers

with on-chip termination resistors only work for point-to-

point applications since multi-drop applications would

require termination resistor for each receiver, reducing the

equivalent termination to RT/n. This would reduce the driver

output swing by n.

Advantages:

1. Reduced device count resulting in reduced board space

and production cost.

2. Reduced reflections caused by the stub length on the

receiver inputs, improving the signal integrity.

Disadvantages:

1. Without special process treatment, on-chip termination can

experience greater temperature variation. This is usually

tolerable for low speed applications that have a sufficient

unit interval.

2. For applications with high common-mode noise, a center

tapped capacitor at the receiver side is desirable to filter

out the common-mode voltage noise of the input LVDS sig-

nal. This scheme works for an external termination scheme

with two (50Ω each for nominal 100Ω termination resistor)

half-value termination resistors connected in series and

center tapped to a capacitor to Ground. To implement this

scheme using internal termination resistors, a center tap

pin would have to be used. This would increase the pack-

age size of the part.

3

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