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涉及
ANALOGINPUT
_
+
_
+
IN−
IN+
ADS8406
_
+
V
CC+
V
CC−
1 k
Ω
1 k
Ω
THS4503
68 pf
68 pf
50
Ω
50
Ω
20 pf
R
G
R
S
R
T
OCM
1 k
Ω
R
g,
R
S
, 和 r
T
应当 是 选择 此类 那
R
G
+
R
S
|| r
T
= 1 k
Ω
V
OCM
= 2 v, +v
CC
= 7 v, 和 −v
CC
= −7 v
ADS8406
SLAS426A–AUGUST2004–REVISEDDECEMBER2004
principlesofoperation(持续)
theanaloginputisprovidedtotwoinputpins:+inand–in.whenaconversionisinitiated,thedifferentialinput
onthesepinsissampledontheinternalcapacitorarray.whileaconversionisinprogress,bothinputsare
disconnectedfromanyinternalfunction.
theads8406canoperatewithanexternalreferencewitharangefrom2.5vto4.2v.a4.096-vinternal
referenceisincluded.wheninternalreferenceisused,pin2(refout)shouldbeconnectedtopin1(refin)
witha0.1-µfdecouplingcapacitorand1-µfstoragecapacitorbetweenpin2(refout)andpins47and48
(refm)(seefigure33).theinternalreferenceoftheconverterisdoublebuffered.ifanexternalreferenceis
使用,thesecondbufferprovidesisolationbetweentheexternalreferenceandthecdac.thisbufferisalso
usedtorechargeallofthecapacitorsofthecdacduringconversion.pin2(refout)canbeleftunconnected
(floating)ifexternalreferenceisused.
whentheconverterenterstheholdmode,thevoltagedifferencebetweenthe+inand-ininputsiscapturedon
theinternalcapacitorarray.both+inand–ininputshavearangeof–0.2vtov
ref
+0.2v.theinputspan(+in
–(–in))islimitedto-v
ref
toV
ref
..
theinputcurrentontheanaloginputsdependsuponanumberoffactors:samplerate,inputvoltage,andsource
阻抗.essentially,thecurrentintotheads8406chargestheinternalcapacitorarrayduringthesample
时期.afterthiscapacitancehasbeenfullycharged,thereisnofurtherinputcurrent.thesourceoftheanalog
inputvoltagemustbeabletochargetheinputcapacitance(25pf)toan16-bitsettlinglevelwithintheacquisition
时间(150ns)ofthedevice.whentheconvertergoesintotheholdmode,theinputimpedanceisgreaterthan1
G
Ω
.
caremustbetakenregardingtheabsoluteanaloginputvoltage.tomaintainthelinearityoftheconverter,这
+inand–ininputsandthespan(+in–(–in))shouldbewithinthelimitsspecified.outsideoftheseranges,这
转换器'slinearitymaynotmeetspecifications.tominimizenoise,lowbandwidthinputsignalswithlow-通过
filtersshouldbeused.
Careshouldbetakentoensurethattheoutputimpedanceofthesourcesdriving+INand–INinputsare
matched.ifthisisnotobserved,thetwoinputscouldhavedifferentsettingtime.thismayresultinoffseterror,
gainerrorandlinearityerrorwhichvarieswithtemperatureandinputvoltage.
atypicalinputcircuitusingti'sths4503isshowninfigure33.inputfromasingle-endedsourcemaybe
convertedintoadifferentialsignalfortheads8406asshowninthefigure.incasethesourceitselfisdifferential,
thentheths4503maybeusedindifferentialinputanddifferentialoutputmodes.
figure33.usingtheths4503withtheads8406
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