Specifications

Section 3. CR3000 Measurement Details
There are two sets of channel numbers on the analog channels.
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.
NOTE
Because a single ended measurement is referenced to CR3000 ground, any
difference in ground potential between the sensor and the CR3000 will result
in an error in the measurement. For example, if the measuring junction of a
copper-constantan thermocouple, being used to measure soil temperature, is
not insulated and the potential of earth ground is 1 mV greater at the sensor
than at the point where the CR3000 is grounded, the measured voltage would
be 1 mV greater than the thermocouple output, or approximately 25
o
C high.
Another instance where a ground potential difference creates a problem is
where external signal conditioning circuitry is powered from the same source
as the CR3000. Despite being tied to the same ground, differences in current
drain and lead resistance result in different ground potential at the two
instruments. For this reason, a differential measurement should be made on an
analog output from the external signal conditioner. Differential measurements
MUST be used when the inputs are known to be different from ground, such as
the output from a full bridge.
Common mode range
In order to make a differential measurement, the inputs must be within the
CR3000 common mode range of
±5 V. The common mode range is the
voltage range, relative to CR3000 ground, within which both inputs of a
differential measurement must lie, in order for the differential measurement to
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 CR3000 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 CR3000 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 use the voltage range option to pull into
common mode range (Section 3.1.1) or 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
CR3000 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
CR3000. 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
CR3000 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 CR3000 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.
3-5