Specifications

Section 3. CR3000 Measurement Details
3.1 Analog Voltage Measurement Sequence
The CR3000 measures analog voltages with either an integrate and hold or
analog to digital (A/D) conversion. The A/D conversion is made with a 16-bit
successive approximation technique which resolves the signal voltage to
approximately one part in 60,000 of the full scale range. Using the fastest
possible measurements at the minimum scan rate of 10 ms (100 Hz) the
CR3000 can make and store measurements from all 14 differential channels.
The maximum conversion rate is 1500 per second for measurements made on a
single channel.
The timing of CR3000 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 CR3000 has 5 fixed voltage ranges and autorange. The 16 bit A/D has a
resolution of 1 part in 2
16
(65,536). 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 60,000 resolution over the FSR. For example,
on the ±5000 mV range the full scale range is 10000 mV [5000 - (-5000)] and
the resolution is one sixth of a millivolt; 10000/0.167 = 60000. 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 CR3000 indicates the overrange by returning Not-A-Number (NAN) for
the measurement.
AutoRange
For signals that do not fluctuate too rapidly, AutoRange allows the CR3000 to
automatically choose the voltage range to use. The CR3000 AutoRange makes
two measurements. The first measurement determines the range to use. It is
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