Campbell CR9000, Instruction Manual

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Section 3. CR9000 Measurement Details
3-3
sets the excitation, delays, measures ,
reverses the excitation, delays, measures ,
reverses the excitation, reverses the inputs, delays, measures ,
reverses the excitation, delays, measures .
Thus there are four delays per channel measured.
3.1.1.3 Integration
With the 9050 and 9055 analog input modules, there is no analog integration
of the signal and minimal filtering from the 422 ohm series resistor and 0.001
µ
F capacitor to ground that protect the input. The signal is sampled when the
task sequencer issues a hold command and any noise that may be on the signal
becomes part of the measured voltage. The rapid sample is a necessity for high
speed measurements. Integrating the signal will reduce noise. When lower
noise measurements are needed or speed is not an issue, integration can be
specified as part of the measurement.
The CR9000 uses digital integration. An integration time in microseconds (10
µ
s resolution) is specified as part of the measurement instruction. The CR9000
will repeat measurements every 10
µ
s throughout the integration interval and
store the average as the result of the measurement.
The random noise level is decreased by the square root of the number of
measurements made. For example, the input noise on the
±
5000 mV range
with no integration (one measurement) is 90
µ
V RMS; integrating for 40
µ
s
(four measurements) will cut this noise in half (90/(
√
4)=45).
One of the most common sources of noise is not random but is 60 Hz from AC
power lines. An integration time of 16,670
µ
s is equal to one 60 Hz cycle.
Integrating for one cycle will integrate the AC noise to 0.
The integration time specified in the measurement instruction is used for each
segment of the measurement. Thus, if reversing the differential input or
reversing the excitation is specified, there will be two integrations per channel;
if both reversals are specified, there will be four integrations.
3.1.2 Single Ended and Differential Voltage Measurements
A single-ended measurement is made on a single input which is measured
relative to ground. A differential measurement measures the difference in
voltage between two inputs. Twice as many single ended measurements can
be made per Analog Input Module.
There are two sets of channel numbers on the Analog Input
Modules. Differential channels (1-14) have two inputs: high (H)
and low (L). Either the high or low side of a differential channel
can be used for a single ended measurement. The single-ended
channels are numbered 1-28.
Because a single ended measurement is referenced to CR9000 ground, any
difference in ground potential between the sensor and the CR9000 will result
NOTE