4116R-R2R-503 datasheet(48/62 Pages) BOURNS

48 / 62 page

Specifications are subject to change without notice.

324

FOR PRODUCT SPECIFICATIONS, SEE PAGES 309 - 311.

Typical Application

A typical application using a Bourns 801 RC Network in con-

junction with a 10K ECL design is shown below. Vee is typically

connected to -5.2 volts (10K ECL) or -4.5 volts (100K ECL). Vcc

is typically connected to GND. Vtt is typically connected to -2.0

volts. The 801 network shown below can terminate up to 6

transmission lines and provides a 0.01

µF capacitor to reduce

cross talk and feedthrough effects.

The actual, effective delay and impedance due to

loading from stubs or additional devices off the line will be:

Tpd’ = Tpd √1 + Cd/Co

Zo’ =

√1 + (C

d/Co)

Where Co = intrinsic capacitance of the line

Cd = capacitance due to loading and stubs

off the line

Tpd = basic propagation delay of the line

Zo ....= basic impedance of the line

To formulate a guideline for when line termination is

necessary, take the ratio of the rise time or fall time and the two-

way delay along the line. The maximum length for unterminated

lines will result as follows:

Lmax =

2Tpd’

Where Tr = rise or fall time

Tpd = propagation delay per unit length

The above equation implies that the faster the edge rate or the

higher the loading on the line (i.e., higher fanout), the more likely

that termination will be necessary for a given line length.

Parallel Termination

For maximum circuit performance or distributed loads, parallel

termination is the most appropriate technique. A parallel termi-

nated line uses a resistor connected to -2 volts (ECL application)

at the receiving end. The resistor value matches the characteris-

tic impedance of the line (Zo), thereby producing zero reflection

at the receiver. In addition, the terminating resistor also provides

output pull down, so a separate pull down resistor at the driving

end is unnecessary.

Transmission Line Considerations

In high speed circuit applications, the signal propagation delay

(Tpd) and characteristic impedance (zo), along a printed circuit

board line must be taken into consideration. In general, if the

two-way delay along the line is greater than the rise or fall time

of the signal, then controlled impedance techniques (i.e., termi-

nation) must be utilized to prevent undesirable ringing or over-

and undershoots. The delay and impedance can be calculated

by knowing the intrinsic inductance (Lo) and capacitance (Co) of

the line:

Tpd = √LoCo

Zo =

√L

o/Co

Vcc1

Aout

A1in

Vee

Vcc2

Bout

B1in

A1in

A2in

B1in

B2in

A2in,

MC10H118 MECL 10KH

Vee

TRANSMISSION

LINES

Vtt

BOURNS 801

10K ECL TERMINATOR

Bourns Emitter Coupled Logic Terminator 800 Series

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