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THEORYOFOPERATION
V
REF
地
I
输出
2R
2R 2R 2R 2R 2R 2R 2R 2R 2R 2R 2R R
FB
R R
R
V
输出
V
REF
代号
65536
(1)
OPA277
DAC8820
V
DD
V
DD
+15 V
U2
地
R
OFS
R
FB
V
输出
I
输出
V
REF
V+
−
15 V
V
−
U1
DAC8820
SBAS358A–AUGUST2005–REVISEDNOVEMBER2005
thedac8820isamultiplying,单独的-channelcurrentoutput,16-bitdac.thearchitecture,illustratedin
Figure17,isanr-2rladderconfigurationwiththethreemsbssegmented.each2rlegoftheladderiseither
switchedtoGNDortotheI
输出
终端.thei
输出
terminaloftheDACisheldatavirtualGNDpotentialbythe
useofanexternali/vconverteropamp.ther-2rladderisconnectedtoanexternalreferenceinput(v
REF
)那
determinesthedacfull-scalecurrent.ther-2rladderpresentsacodeindependentloadimpedancetothe
externalreferenceof6k
Ω±
25%.theexternalreferencevoltagecanvaryinarangeof–15vto+15v,因此
providingbipolarI
输出
currentoperation.byusinganexternali/vconverteropampandthedac8820r
FB
电阻,anoutputvoltagerangeof–v
REF
to+V
REF
canbegenerated.
figure17.equivalentr-2rdaccircuit
whenusinganexternali/vconverteropampandthedac8820r
FB
电阻,thedacoutputvoltageisgivenby
Equation1:
eachdaccodedeterminesthe2r-legswitchpositiontoeithergndori
输出
.theexternali/vconverteropamp
noisegainwillalsochangebecausethedacoutputimpedance(asseenlookingintothei
输出
终端)改变
versuscode.becauseofthis,theexternali/vconverteropampmusthaveasufficientlylowoffsetvoltagesuch
thattheamplifieroffsetisnotmodulatedbytheDACI
输出
terminalimpedancechange.externalopampswith
largeoffsetvoltagescanproduceINLerrorsinthetransferfunctionoftheDAC8820becauseofoffset
modulationversusdaccode.forbestlinearityperformanceofthedac8820,anopamp(opa277)是
推荐,asshowninFigure18.thiscircuitallowsv
REF
toswingfrom–15vto+15v.
figure18.voltageoutputconfiguration
8
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