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MULTIPLEXERINPUTS
AVSS
100mV
VREFP 或者 VREFN
AVDD
100mV
(1)
VOLTAGEREFERENCEINPUTS
静电释放
二极管
静电释放
二极管
3pF
R
eff
= 40k
Ω
(f
CLK
= 16mhz)
AVDD
AVSS
VREFP
VREFN
ADS1258
SBAS297A–JUNE2005–REVISEDSEPTEMBER2005
esddiodesprotectthereferenceinputs.tokeep
thesediodesfromturningon,makesurethevoltages
Asimplifieddiagramoftheinputmultiplexeris
onthereferencepinsdonotgobelowAVSSbymore
illustratedinFigure35.themultiplexerconnectsone
than100mv,andlikewisedonotexceedavddby
of16single-endedexternalinputs,oneofeight
100mv,asdescribedinEquation1:
differentialexternalinputs,oroneoftheon-碎片
internalvariablestotheadcinputs.theoutputofthe
channelmultiplexercanberoutedtoexternalpins
ahigh-qualityreferencevoltageisessentialfor
andthentotheinputoftheadc.thisflexibility
achievingthebestperformancefromtheads1258.
allowsforuseofexternalsignalconditioning.seethe
Noiseanddriftonthereferencedegradeoverall
ExternalMultiplexerLoop
部分.
systemperformance.itisespeciallycriticalthat
specialcarebegiventothecircuitrythatgenerates
esddiodesprotecttheanaloginputs.tokeepthese
thereferencevoltagesandthelayoutwhenoperating
diodesfromturningon,makesurethevoltageson
inthelow-noisesettings(thatis,withlowdatarates)
theinputpinsdonotgobelowAVSSbymorethan
topreventthevoltagereferencefromlimiting
100mv,andlikewisedonotexceedavddbymore
效能.seethe
ReferenceInputs
descriptionin
than100mv:
这
HardwareConsiderations
segmentofthe
avss–100mv<(analoginputs)<avdd+100mv.
产品
部分.
Theconvertersupportstwomodesofchannelaccess
throughthemultiplexer:theauto-scanmodeandthe
fixed-channelmode.thesemodesareselectedby
themuxmodbitofregisterconfig0.这
自动-scanmodescansthroughtheselected
channelsautomatically,withbreak-在之前-制造
切换.thefixed-channelmoderequirestheuser
tosetthechanneladdressforeachchannel
量过的.
(vrefp,vrefn)
ThevoltagereferencefortheADS1258ADCisthe
differentialvoltagebetweenvrefpandvrefn:
V
REF
=vrefp–vrefn.thereferenceinputsusea
structuresimilartothatoftheanaloginputswiththe
circuitryonthereferenceinputsshowninFigure34.
Theloadpresentedbytheswitchedcapacitorcanbe
modeledwithaneffectiveresistance(r
eff
)of40k
Ω
为
f
CLK
=16mhz.notethattheeffectiveimpedanceof
figure34.simplifiedreferenceinputcircuit
thereferenceinputswillloadanexternalreference
withanon-zerosourceimpedance.
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