Operator`s manual

SECTION 13. 21X MEASUREMENTS
13-2
averaging the magnitude of the results from the
two integrations and using the polarity from the
first. An exception to this is the differential
measurement in Instruction 8 which makes only
one integration.
FIGURE 13.2-1. Differential Voltage
Measurement Sequence
Because a single-ended measurement is
referenced to 21X ground, any difference in
ground potential between the sensor and the
21X 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 1mV greater at
the sensor than at the point where the 21X is
grounded, the measured voltage would be 1mV
greater than the thermocouple output or
approximately 25°C 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 21X. 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 where the inputs are known to be
different from ground, such as is the case with
the output from a full bridge.
In order to make a differential measurement,
the inputs must be within the 21X common
mode range of ±5V. The common mode range
is the voltage range, relative to 21X 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 21X
ground, there is no problem; a measurement
made on the ±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
cannot be made because the high input is
outside of the common mode range. (The 21X
will indicate the overrange with the maximum
negative number, see Section 3.5.)
Problems with exceeding common mode range
may be encountered when the 21X 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 21X.
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 21X 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 21X
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 21X. Input
offset voltage on a single-ended measurement
is less than 5 microvolts; the input offset voltage
on a differential measurement is less than 1
microvolt.
A single-ended measurement is quite
satisfactory in cases where noise is not a
problem and care is taken to avoid ground
potential problems. Channels are available for
twice as many single-ended measurements. 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 21X input circuitry.