www.德州仪器.com
ADS7888NORMALOPERATION
1
2 4
5
6
12
15
CS
SCLK
SDO
0
0 0
D7 D6
D1
D0
11
0
0
16
t
d1
t
d2
t
h1
t
conv
1/throughput
t
q
t
d3
t
w1
b
t
su1
一个
0
1
2 4
5
6
12
CS
SCLK
SDO
0
0 0
D7 D6
D1
D0
11
t
d1
t
d2
t
h1
t
conv
1/throughput
t
q
t
h1
t
w1
b
t
su1
一个
t
d4
POWERDOWNMODE
ADS7887
ADS7888
SLAS468–JUNE2005
thecyclebeginswiththefallingedgeofcs.thispointisindicatedas
一个
在Figure2.withthefallingedgeofcs,
theinputsignalissampledandtheconversionprocessisinitiated.thedeviceoutputsdatawhiletheconversion
isinprogress.thedatawordcontains4leadingzeros,followedby8-bitdatainmsbfirstformatandpaddedby
4laggingzeros.
thefallingedgeofcsclocksoutthefirstzero,andazeroisclockedoutoneveryfallingedgeoftheclockuntil
thethirdedge.dataisinmsbfirstformatwiththemsbbeingclockedoutonthe4thfallingedge.dataispadded
withfourlaggingzerosasshowninFigure2.onthe16thfallingedgeofsclk,sdogoestothe3-状态
情况.theconversionendsonthe12thfallingedgeofsclk.thedeviceenterstheacquisitionphaseonthe
firstrisingedgeofsclkafterthe11thfallingedge.thispointisindicatedby
b
在Figure2.
cscanbeasserted(pulledhigh)after16clockshaveelapsed.itisnecessarynottostartthenextconversionby
pullingcslowuntiltheendofthequiettime(t
q
)aftersdogoesto3-状态.tocontinuenormaloperation,itis
necessarythatCSisnotpulledhighuntilpoint
b
.withoutthis,thedevicedoesnotentertheacquisitionphase
andnovaliddataisavailableinthenextcycle.(alsorefertopowerdownmodeformoredetails.)csgoinghigh
anytimeaftertheconversionstartabortstheongoingconversionandsdogoesto3-状态.
ThehighlevelofthedigitalinputtothedeviceisnotlimitedtodeviceV
DD
.thismeansthedigitalinputcangoas
highas5.25vwhenthedevicesupplyis2.35v.thisfeatureisusefulwhendigitalsignalsarecomingfrom
anothercircuitwithdifferentsupplylevels.也,thisrelaxestherestrictiononpowerupsequencing.不管怎样,
thedigitaloutputlevels(v
OH
andV
OL
)aregovernedbyv
DD
aslistedinthespecificationssection.
figure2.ads7888interfacetimingdiagram
AsshowninFigure3,theads7888canachieve1.5-mspsthroughput.cscanbepulledhighafterthe12th
fallingedge(witha25-mhzsclk).sdogoesto3-stateafterthelsb(ascsishigh).cscanbepulledlowat
theendofthequiettime(t
q
)aftersdogoesto3-状态.
figure3.ads7888interfacetimingdiagram,datatransferwith12-clockframe
ThedeviceenterspowerdownmodeifCSgoeshighanytimeafterthe2ndSCLKfallingedgetobeforethe10th
sclkfallingedge.ongoingconversionstopsandsdogoesto3-stateunderthispowerdownconditionas
showninFigure4.
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