www.德州仪器.com
FAILSAFECONSIDERATIONS
1.6 k
Ω
1.6 k
Ω
100
Ω
3.3 v
sn65lvds100,sn65lvdt100
sn65lvds101,sn65lvdt101
SLLS516C–AUGUST2002–REVISEDJUNE2004
applicationinformation(持续)
failsafe,inregardtoalinereceiver,usuallymeansthattheoutputgoestoadefinedlogicalstatewithnoinput
信号.tokeepaddedjittertoanabsoluteminimum,thesn65lvds100doesnotincludethisfeature.itdoes
exhibit25mVofinputvoltagehysteresistopreventoscillationandkeeptheoutputinthelaststatepriorto
输入-signalloss(assumingthedifferentialnoiseinthesystemislessthanthehysteresis).
shouldfailsafeberequired,itmaybeaddedexternallywitha1.6-k
Ω
拉-upresistortothe3.3-vsupplyanda
1.6-k
Ω
拉-downresistortogroundasshowninfigure45thedefaultoutputstateisdeterminedbywhichlineis
pulledupordownandistheuser'schoice.thelocationofthe1.6-k
Ω
resistorsisnotcritical.howeverthe100-
Ω
resistorshouldbelocatedattheendofthetransmissionline.
figure45.externalfailsafecircuit
additionofthisexternalfailsafewillreducethedifferentialnoisemarginandaddjittertotheoutputsignal.这
roughly100-mvsteady-statevoltagegeneratedacrossthe100-
Ω
resistoradds(orsubtracts)fromthesignal
generatedbytheupstreamlinedriver.ifthelinedriver'sdifferentialoutputissymmetricalaboutzerovolts,然后
theinputatthereceiverwillappearasymmetricalwiththeexternalfailsafe.perhapsmoreimportant,istheextra
timeittakesfortheinputsignaltoovercometheaddedfailsafeoffsetvoltage.
infigure46andusinganexternalfailsafe,thehigh-leveldifferentialvoltageattheinputofthesn65lvds100
reaches340mvandthelow-level–400mvindicatinga60-mvdifferentialoffsetinducedbytheexternalfailsafe
电路系统.thefigurealsorevealsthatthelowestpeak-至-peaktimejitterdoesnotoccuratzero-voltdifferential
(thenominalinputthresholdofthereceiver)butat–60mv,thefailsafeoffset.
theaddedjitterfromexternalfailsafeincreasesasthesignaltransitiontimesareslowedbycableeffects.当
aten-metercat-5utpcableisintroducedbetweenthedriverandreceiver,thezero-crossingpeak-至-peakjitter
atthereceiveroutputadds250pswhentheexternalfailsafeisaddedwiththisspecifictestsetup.ifexternal
failsafeisusedinconjunctionwiththesn65lvds100,thenoisemarginandjittereffectsshouldbebudgeted.
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