Campbell CR800 Specifikace stáhnout pdf (Strana 75)

3-1

Section 3. CR800 Measurement Details

3.1 Analog Voltage Measurement Sequence

The CR800 measures analog voltages with either an integrate and hold or

analog to digital (A/D) conversion. The A/D conversion is made with a 13-bit

successive approximation technique which resolves the signal voltage to

approximately one part in 7500 of the full scale range. Using the fastest

possible measurements at the minimum scan rate of 10 µs (100 Hz) the CR800

can make and store measurements from all 3 differential or 6 single-ended

channels. The maximum conversion rate is 2700 per second for measurements

made on a single channel.

The timing of CR800 measurements is precisely controlled. The measurement

schedule is determined at compile time and loaded into memory. This

schedule sets interrupts that drive the measurement task.

Using two different voltage measurement instructions with the same

voltage range takes the same measurement time as using one instruction

with two repetitions. (This is not the case in the CR10X, 21X, CR23X and

CR7 dataloggers where there is always a setup time for each instruction.)

There are four parameters in the measurement instructions that may vary the

sequence and timing of the measurement. These are options to measure and

correct the ground offset on single-ended measurements each time

measurements are made (MeasOfs), to reverse the high and low differential

inputs (RevDiff), to set the time to allow the signal to settle between switching

to a channel and making a measurement (SettlingTime), to set the length of

time to integrate a measurement (Integ), and to reverse the polarity of

excitation voltage (RevEx).

3.1.1 Voltage Range

Fixed Voltage Ranges

The CR800 has 6 fixed voltage ranges and autorange. The 13 bit A/D has a

resolution of 1 part in 2

(8,192). To allow for some overrange capabilities

the A/D is applied to a range approximately 9% greater than the Full Scale

Range resulting in the 1 part in 7500 resolution over the FSR. For example, on

the ±2500 mV range the full scale range is 5000 mV [2500 - (-2500)] and the

resolution is two thirds of a millivolt; 5000/.667 = 7500. The smaller the

voltage range, the better the absolute resolution. In general, a measurement

should use the smallest fixed voltage range that will accommodate the full

scale output of the sensor being measured. If the voltage exceeds the range,

the CR800 indicates the overrange by returning Not-A-Number (NAN) for the

measurement.

AutoRange

For signals that do not fluctuate too rapidly, AutoRange allows the CR800 to

automatically choose the voltage range to use. The CR800 AutoRange makes

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