LTC2309
2309fd
AppLICAtIOns InFORMAtIOn
WhenusingafilterwithalargeCFILTERvalue(e.g.1F),
theinputsdonotcompletelysettleandthecapacitive
inputswitchingcurrentsareaveragedintoanetDC
current(IDC).Inthiscase,theanaloginputcanbemod-
eledbyanequivalentresistance(REQ=1/(fSMPLCIN))
inserieswithanidealvoltagesource(VREFCOMP/2),as
showninFigure3b.ThemagnitudeoftheDCcurrent
isthenapproximatelyIDC=(VIN–VREFCOMP/2)/REQ,
whichisroughlyproportionaltoVIN.Topreventlarge
DCdropsacrosstheresistorRFILTER,afilterwithasmall
resistorandlargecapacitorshouldbechosen.When
runningatthemaximumthroughputrateof14ksps,
theinputcurrentequals1.5AatVIN=4.096V,which
amountstoafull-scaleerrorof0.5LSBwhenusinga
filterresistor(RFILTER)of333.Applicationsrequiring
lowersampleratescantoleratealargerfilterresistor
forthesameamountoffull-scaleerror.
selfheatingandfromdamagethatmayoccurduring
soldering.Metalfilmsurfacemountresistorsaremuch
lesssusceptibletobothproblems.
Dynamic Performance
FastFourierTransform(FFT)testtechniquesareusedto
testtheADC’sfrequencyresponse,distortionandnoise
attheratedthroughput.Byapplyingalowdistortion
sinewaveandanalyzingthedigitaloutputusinganFFT
algorithm,theADC’sspectralcontentcanbeexamined
forfrequenciesoutsidethefundamental.
Signal-to-Noise and Distortion Ratio (SINAD)
Thesignal-to-noiseanddistortionratio(SINAD)isthe
ratiobetweentheRMSamplitudeofthefundamental
inputfrequencytotheRMSamplitudeofallotherfre-
quencycomponentsattheA/Doutput.Theoutputis
band-limitedtofrequenciesfromaboveDCandbelow
halfthesamplingfrequency.Figure5showsatypical
SINADof73.3dBwitha14kHzsamplingrateanda
1kHzinput.AnSNRof73.4dBcanbeachievedwith
theLTC2309.
VIN
INPUT
CH0-CH7
RON
100
CIN
55pF
CFILTER
RSOURCE
2309 F03a
LTC2309
Figure 3a. Analog Input Equivalent Circuit
VIN
INPUT
CH0-CH7
REQ
1/(fSMPL CIN)
VREFCOMP/2
CFILTER
RFILTER IDC
2309 F03b
LTC2309
+–
Figure 3b. Analog Input Equivalent
Circuit for Large Filter Capacitances
Figures4aand4bshowexamplesofinputfilteringfor
single-ended and differential inputs. For the single-
endedcaseinFigure4a,a50sourceresistoranda
2000pFcapacitortogroundontheinputwilllimitthe
inputbandwidthto1.6MHz.Highqualitycapacitorsand
resistorsshouldbeusedintheRCfiltersincethese
componentscanadddistortion.NPOandsilvermica
typedielectriccapacitorshaveexcellentlinearity.Carbon
surfacemountresistorscangeneratedistortionfrom
2309 F04a
CH0
COM
LTC2309
REFCOMP
2000pF
0.1F
10F
50
ANALOG
INPUT
Figure 4a. Optional RC Input Filtering for Single-Ended Input
1000pF
2309 F04b
CH0
CH1
LTC2309
REFCOMP
1000pF
0.1F
10F
50
DIFFERENTIAL
ANALOG
INPUTS
Figure 4b. Optional RC Input Filtering for Differential Inputs