Campbell CR800 Spécifications télécharger pdf (Page 77)

Section 3. CR800 Measurement Details

3-3

3.1.2 Reversing Excitation or the Differential Input

Reversing the excitation polarity or the differential input are techniques to

cancel voltage offsets that are not part of the signal. For example, if there is a

+5 µV offset in the measurement circuitry, a 5 mV signal will be measured as

5.005 mV. When the input is reversed, the measurement will be

-4.995 mV. Subtracting the second measurement from the first and dividing

by 2 gives the correct answer: 5.005-(-4.995)=10, 10/2=5. Most offsets are

thermocouple effects caused by temperature gradients in the measurement

circuitry or wiring.

Reversing the excitation polarity cancels voltage offsets in the sensor, wiring,

and measurement circuitry. One measurement is made with the excitation

voltage with the polarity programmed and a second measurement is made with

the polarity reversed. The excitation "on time" for each polarity is exactly the

same to ensure that ionic sensors do not polarize with repetitive measurements.

Reversing the inputs of a differential measurement cancels offsets in the

CR800 measurement circuitry and improves common-mode rejection. One

measurement is made with the high input referenced to the low input and a

second with the low referenced to the high.

3.1.3 Measuring Single-Ended Offset

The single-ended offset is a voltage offset on a single-ended input. It is

measured by internally switching the input to ground and measuring the

voltage. When a single-ended measurement is made this offset is corrected for

in the calibration. The offset can either be measured automatically as part of

the background calibration or as part of the measurement sequence each time

the measurement is made (adding to the time to make the measurement).

When the offset is measured in the measurement sequence, the offset is

measured once prior to completing all of the instruction reps.

The MeasOfs parameter in instructions that make single-ended voltage

measurements is used to force the offset measurement. In most cases the

background calibration is adequate. Additional accuracy can be gained by

making the offset measurement with each measurement instruction when the

offset is changing rapidly as it would during when the CR800 is undergoing

rapid temperature swings.

3.1.4 SettlingTime

When the CR800 switches to a new channel or switches on the excitation for a

bridge measurement, there is a finite amount of time required for the signal to

reach its true value. Delaying between setting up a measurement (switching to

the channel, setting the excitation) and making the measurement allows the

signal to settle to the correct value. The default settling times are the

minimum required for the CR800 to settle to within its accuracy specifications.

Additional time is necessary when working with high sensor resistances or

long lead lengths (higher capacitance). Using a longer settling time increases

the time required for each measurement. Section 3.3 goes into more detail on

determining an adequate settling time.

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