Campbell-scientific CR3000 Micrologger Manual de usuario descargar pdf (Pagina 304)

100

101

Section 8. Operation

304



• Offset = 3 x Basic Resolution + 5.0 µV if the measurement is of a single-

ended input channel

Thefollowingtablelistsbasicresolutionvalues.

Table 65. Analog Input-Voltage Range and Basic

Resolution

Range (mV) Basic Resolution (µV)

±5000

±1000

±200

±50

±20

167

33.3

6.67

1.67

0.67

Assumptions that support the ratiometric-accuracy specification include:

•

• Excitation voltages less than 500 mV and excitation currents less than 500

µA are reversed during the excitation phase of the measurement.

• Effects due to the following are not included in the specification:

o Bridge-resistor errors

o Sensor noise

o Measurement noise

The ratiometric-accuracy specification is applied to a three-wire half-bridge

measurement that uses the BrHalf() instruction as follows:

TherelationshipdefiningtheBrHalf()instructionisX=V1/Vx,whereV1

isthevoltagemeasurementandVxistheexcitationvoltage.The

estimatedaccuracyofXisdesignatedas∆X,where∆X=∆V1/Vx.∆V1is

derivedusingth

efollowingmethod.

The ratiometric-accuracy specification is applied to a four-wire full-bridge

measurement that uses the BrFull() instruction as follows:

TherelationshipdefiningtheBrFull()instructionisX=1000*V1/Vx,

whereV1isthevoltagemeasurementandVxistheexcitationvoltage.

ResultXisexpressedasmV/V.EstimatedaccuracyofXis∆X,where∆X

=1000*∆V1/Vx.∆V1isderiv

edusingthefollowingmethod.

∆V1isderivedusingtheratiometric‐accuracyequation.Thederivation

isillustratedinthisexample,whichissupportedbytheassumptionthat

themeasurementisdifferentialwithinputreversal,datalogger

temperatureisbetween0°to40°C,analog‐inputrangeis±200mV,V1=

0mV,andexcitationisreversedduringtheexcitationphaseofthe

measurement.Theeffecteachassumptionhasonthemagnitudeof

∆V1inthisexampleisnotedinthefollowingfigure.

In the case of the Resistance() instruction, sensor resistance is determined from

Vs/Ix, where excitation current Ix is measured across a 1000 , ±0.005% @ 25

˚C, 2 ppm/˚C TCR internal resistor.

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