Campbell CR5000, Руководство оператора

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Section 3. CR5000 Measurement Details
3-5
be made. For example, if the high side of a differential input is at 4 V and the
low side is at 3 V relative to CR5000 ground, there is no problem. A
measurement made on the
±
5000 mV range will return 1000 mV. However, if
the high input is at 5.8 V and the low input is at 4.8 V, the measurement can
not be made because the high input is outside of the
±
5 V common mode
range (the CR5000 will indicate the overrange by returning not-a-number
(NAN)).
Sensors that have a floating output or are not referenced to ground through a
separate connection may need to have one side of the differential input
connected to ground to ensure the signal remains within the common mode
range.
Problems with exceeding common mode range may be encountered when the
CR5000 is used to read the output of external signal conditioning circuitry if a
good ground connection does not exist between the external circuitry and the
CR5000. When operating where AC power is available, it is not always safe to
assume that a good ground connection exists through the AC wiring. If a
CR5000 is used to measure the output from a laboratory instrument (both
plugged into AC power and referencing ground to outlet ground), it is best to
run a ground wire between the CR5000 and the external circuitry. Even with
this ground connection, the ground potential of the two instruments may not be
at exactly the same level, which is why a differential measurement is desired.
A differential measurement has the option of reversing the inputs to cancel
offsets as described above.
Sustained voltages in excess of
±
16 V will damage the CR5000
circuitry.
3.3 Signal Settling Time
Whenever an analog input is switched into the CR5000 measurement circuitry
prior to making a measurement, a finite amount of time is required for the
signal to stabilize at it's correct value. The rate at which the signal settles is
determined by the input settling time constant which is a function of both the
source resistance and input capacitance.
The CR5000 delays after switching to a channel to allow the input to settle
before initiating the measurement. The default delays used by the CR5000
depend on the integration used and the voltage range. For the sample-and-
hold, the default delay is 100 µs on the ±5000, ±1000, ±200, and ±50 mV
ranges and 200 µs on the ±20 mV range. The default delay is 200 µs for 250
µs integrations and 3 ms for 50 Hz or 60 Hz rejection integrations. This
settling time is the minimum required to allow the input to settle to the
resolution specification.
Additional wire capacitance associated with long sensor leads can increase the
settling time constant to the point that measurement errors may occur. There
are three potential sources of error which must settle before the measurement is
made:
NOTE