Campbell CR23X, Руководство оператора

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SECTION 13. CR23X MEASUREMENTS
13-3
250 us fast 260 us 500 us 250 us fast 260 us
16.67 ms 60 Hz Reject 16.67 ms 60 Hz Reject
20.00 ms 50 Hz Reject 20.00 ms 50 Hz Reject
FIGURE 13.2-2. Differential Voltage Measurement Sequence
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 external source as the CR23X. 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 CR23X common
mode range of ±5.0 V. The common mode
range is the voltage range, relative to CR23X
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 CR23X ground, there is no problem; a
measurement made on the
±
5.0 V range would
indicate a signal of 1 V. However, if the high
input is at 5.8 V and the low input is at 4.8 V, the
measurement cannot be made because the high
input is outside of the common mode range.
The CR23X will indicate the overrange with the
maximum negative number (Section 3.5.)
Problems with exceeding common mode range
may be encountered when the CR23X 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
CR23X. 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 CR23X 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 CR23X 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 (Section 7.2).
If a differential measurement is used on a sensor
that is not referenced to CR23X ground through a
separate connection (e.g., a net radiometer), a
jumper wire should be connected between the low
side of the differential input and analog ground to
hold the sensor in common mode range.
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 CR23X.
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 CR23X input circuitry.