XITRON XT2640 - Channel Circuitry; Sampling Control; Channel Measurements

XT2640OperatingManual 13July2016 Page161of187
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25.2 CHANNELCIRCUITRY
Thediagrambelowisasimplifiedblockdiagramofthecircuitryforeachchannel.Upto4channelscanbeinstalledinaXT2640.
Althoughnotshowninthisdiagram,thereisdigitalfilteringonthesampledataforbothvoltageandcurrentasrequiredforthe
BANDWIDTHconfigurationofthecontrollingVPAandthedataisscaledandoffsetaccordingtothechannelcalibrationdataandto
usersetscaleandoffsetfactors.Theanalogfilteringshowninthediagramissimplytoreducesamplingartifacts.
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25.3 SAMPLINGCONTROL
Thereare3samplingcontrolsinaXT2640.TypicallyeachisassociatedwithaVPA,howeverifyouconfiguretheLF/PERIODsetting
inaVPAtosynchronizethatVPAtoalowernumberedVPAthenthesamplingcontrolofthatVPAisusedforthechannelsintheVPA
soconfigured.
Samplingcontrolperformsthefollowing–
Calculationofthemeasurementperiodformeasurementresults.
Themeasurementperiodisadjustedtobeanintegernumberoffundamentalcycleswhichistheclosesttotheconfigured
measurementperiodforthecontrollingVPA.
Ahalf‐cycleperiodisalsogeneratedforcontrollingtheperiodofhalf‐cyclevalleymeasurements.
Calculationandcontrolofthesamplingperiodofchannels.
Therequiredsamplingperiodiscalculatedtoyieldanintegernumberofsamplesinameasurementperiodwhichgivesthe
closestsamplingperiodtothenominalsamplingperiodforthechannelsbeingcontrolled.Thesamplingperiodisalso
adjustedtonotyieldanintegernumberofsamplesinanyharmonic(includingthefundamental)over500Hz.Thenominal
samplingperiodforthechanneltypesareasfollows(thisdoesnotlimittheHFamplitudeperformanceoftheXT2640)–
L,SorAtypes:245KSPS
Wtype:910KSPS
Eachactualsamplingperiodiscontrolledwith2.5psaverageresolutionand2.6nsincrementalresolutionwithaquasi‐
randomdistributionofincrementalsampleperiodswithinarangesufficienttoavoidNyquistsamplinglimits.
Thisyieldsaneffectivesamplingrateof384MSPSandsynchronizationtothemeasuredfundamentaltowithin<1ppm.
Startingandstoppingmeasurementperiods.
IftheXT2640measurementmodeisconfiguredtoanythingotherthanSYNC‐VPAtheneachsamplingcontrolalways
immediatelystartsameasurementperiodwhenthepreviousoneends.InthismannerallchannelandVPAmeasurements
aretotally‘gapless’.
IftheXT2640measurementmodeisconfiguredtoSYNC‐VPAtheneachsamplingcontrolwillstartallmeasurement
periodsifallmeasurementperiodshaveended.InthismannerallchannelandVPAmeasurementsaresynchronized,but
onlythelongestoneis‘gapless’.
25.4 CHANNELMEASUREMENTS
Foreachofthechannelsthefollowingmeasurementsareperformed–
VoltageandcurrentDC(theaverageofthesamplesinthemeasurementperiod).
Voltageandcurrentmaximumpeak(thehighestsampleinthemeasurementperiod).
Voltageandcurrentminimumpeak(thelowestsampleinthemeasurementperiod).
Voltageandcurrenthalf‐cyclebasedvalleymeasurements.
Voltageandcurrentrectified(theaveragemodulussampleinthemeasurementperiod).
SHUNT
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S:22bitADC
A:24bitADC
W:22bitADC
ISOLATION
H:0.0025Ω
D(hi):0.004Ω
D(lo):0.5Ω
X(hi):20.5kΩ
X(lo):10.25kΩ
ATTEN
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S:22bitADC
A:24bitADC
W:22bitADC
ISOLATION
S:1.2MΩ
A:1.2MΩ
W:400kΩ
L:121kΩ
CURRENT
VOLTAGE
DATA
DATA
SAMPLE