SGS Thomson Microelectronics RCD16-47B, RCD16-47B6 Datasheet



RCD16-47B

Application Specific Discretes

A.S.D.

MAINAPPLICATIONS

In any electronic equipment where a suitable bus
terminationis requiredtoavoidsignalreflectionsand
distortions:

PC and workstationcomputer
Data-lineanalyzers

DESCRIPTION

With the increasing speed of data transmission,
line reflections provide signal distorsions and the
overshootsor undershootsproducedon the signal
edges can cause the malfunction of the whole
system.

To avoid these negative effects from leading to
problems, a suitable termination is required.
Dedicated to bus termination, the RCD16-47B
provides by far the best method to minimise stray
emissionsfrom PCB tracks.

FEATURES

NETWORKOF16R-C-DLINETERMINATIONS
RESISTANCE:R = 47
CAPACITANCE: C=33pF,TOLERANCE+/-10%
SCHOTTKY DIODE: (D)

TM

Ω

, TOLERANCE+/-10%

RCD NETWORK

FOR BUS TERMINATION

SO20

SSOP20

FUNCTIONAL DIAGRAM

1

2

20

19

BENEFITS

Provides impedance matching, thus increasing
noiseimmunityand minimizingdistortion.

LowersEMI / RFI radiation.
No DC powerdissipation.
Eliminates negative voltages : no current will

changethebias of the protecteddevice.
Usesthe best of allterminationschemes.

COMPLIESWITHTHEFOLLOWINGSTANDARDS:

MILSTD 883C - Method 3015-6

= 2 kV C=100 pF R = 1500 Ω

-V

PP

-3 positivestrikesand 3 negativestrikes (F=1Hz)

October 1998 - Ed : 2A

3

4

5

6

7

8

9

10

R=47 Ω, tolerance +/−

10

%

C= 33 pF, tolerance +/- 10 %
D= Schottkydiode

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RCD16-47B

APPLICATION NOTE : BUS TERMINATION

Withtheincreasingspeedof datatransmission(PC,TV,...),engineersare naturallyconfrontedwith effects
that were of less significance with slowercircuits. Amongthese are the effectsdescribedin transmission
linetheory : line reflectionsprovide signal distortionsand the overshootsor undershootsproducedon the
signaledges can finally cause themalfunctionof thewhole system.

1. Reflectionat a non terminatedline

Thefigure below showsthe circuitof a transmissionsystemin whicha memorydevice(input impedance of
100kΩ) is connectedat the endof a line with lineimpedanceZo.

10 Ω

100 kΩ

If thisline is notproperlyterminated, a certain amount of the energy isreflected back,inducing a reflection
phenomenathat can distortthe signal.This can resultin improper operation ofthe system.

Thesimulation shownon the abovefigure illustratesthe signal distortionproducedby line reflection at the
endof theline whichis not well terminated.

Evenif thesignalat thestartof theline hasthe correctform(uppercurve),considerabledistortion arisesat
theend of the line (lower curve). On the positive edge, the overshootcan exceed themaximumoperating
voltageof the usedcircuittechnologywhichwill then bedestroyed.Also, the following negativeundershoot
mayreach a level low enoughtochangethe value of thelogicstate. If it affectsof an addressline, a wrong
memorycell will be addressed,and in the caseof adata line, the data can be corrupted.

Thisphenomenaalso occurson thefalling edge.

To avoid these negative effects from leading to problems in a system, a suitable termination is
required.

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RCD16-47B

2. The RCD termination

The traditional solution to properly match each line of the bus consists of the use of several discrete
resistances,capacitorsand smallschottky diodes. For a 16-bit bus, thisrequires48 discretecomponents.

TM

SGS-THOMSONoffers an innovatingsolutionwith a monolithicstructureusing

TM

ASD

The
device. The

technologyenablestointegratemonolithically16 oftheseRCD ”basiccells”onto a singlechip

RCD16-47B

reduces component cost and assemblycost, saves board space and improves

reliability.

ASD

technology(*).

Thesimulationillustratesthe signal distortionproduced by linereflectionat the endof the line when sucha
terminationis used.

The resistorprovidesthe pathterminationfor PCB track,thusresulting inlow reflectionphenomena.
The capacitor of 33 pF blocks DC currents while acting as a shortcircuit during signaltransitions,and

holdsthe busat the last logic level. It reducespowerconsumptionand avoidsexcessivecurrent.
The small Schottkydiode clampsthe negativeremainingundershoots whichcanresultfrom impedance
mismatch.This dampsnegative voltages and preventsthe logic signal from rising above the logic level

’0’ thresholdafter a falling edge.

TheRCD terminationprovides optimal solution compared to all other terminationtechniques.

(*)ASD = ApllicaionSpecificDiscretes.

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