Campbell Scientific CR7 Measurement and Control System - Manual (Page 126)

SECTION 13. CR7 MEASUREMENTS
13-2
FIGURE 13.2-1. Differential Voltage
Measurement Sequence
Because a single ended measurement is
referenced to CR7 ground, any difference in
ground potential between the sensor and the
CR7 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 CR7 is
grounded, the measured voltage would be 1 mV
greater than the thermocouple output, or
approximately 25 oC high. Another instance
where a ground potential difference creates a
problem is in a case such as described in
Section 7.2, where external signal conditioning
circuitry is powered from the same source as
the CR7. 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 the choice where the inputs are known to be
different from ground, such as the output from a
full bridge.
In order to make a differential measurement,
however, the inputs must be within the CR7
common mode range of ±5V. The common
mode range is the voltage range, relative to
CR7 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 4V and the low side is at 3V relative to CR7
ground, there is no problem, a measurement
made on the ±1.5V range would indicate a
signal of 1V. However, if the high input is at
5.8V and the low input is at 4.8V, the
measurement can not be made because the
high input is outside of the CR7 common mode
range (the CR7 will indicate the overrange with
the maximum negative number, Section 2.2).
Problems with exceeding common mode range
may be encountered when the CR7 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
CR7. 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 CR7 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 CR7 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 better noise
rejection than a single ended measurement.
Integrating the signal in both directions also
reduces input offset voltage due to thermal
effects in the amplifier section of the CR7. Input
offset voltage on a differential measurement is
on the order of 0.1 microvolts, the input offset
voltage on a single ended measurement may be
as high as 1 to 2 microvolts.
A single ended measurement is quite
satisfactory in cases where noise is not a
problem and care is taken to avoid ground
potential problems. Twice as many single
ended measurements can be made per Analog
Input Card. A single ended measurement takes
about half the time of a differential
measurement which is valuable in cases where
rapid sampling is a requirement.
NOTE: Sustained voltages in excess of
±16 VDC applied to the analog inputs will
damage the CR7 input circuitry.