Campbell Scientific CR9000X Measurement and Control System - Manual (Page 93)

Section 3. CR9000X Measurement Details
Input Limits
The Input Limit specifies the voltage range, relative to CR9000X ground,
which both H and L input voltages must be within in order to be processed
correctly by the instrumentation amplifier. The Input Limits for the
CR9050(E) and CR9051E modules are
± 5 V . The Input Limits for the
CR9055(E) modules are ± 50 V. Differential measurements in which the H or
L input voltages are beyond the INPUT LIMITs may suffer from undetected
measurement errors.
Example 3.1.2-2: Lets take the case of a type K thermocouple
at about 246 degrees C (thermoelectric voltage of 10 mV) that is
floating with a static charge of 1000 mV. In this case, V
cm
= 1000
mV, V
dm
= 10 mV, V
H
= 995 mV, and V
L
= 1005 mV. A valid
measurement can be made using the mV50 range code because the
1000 mV static charge is within the common mode range, the Diff
voltage is below 50 mV, and the total voltage on both the H (V
H
)
and L (V
L
)inputs are within the ± 5 V Input Limits of the
CR9050.
It should be noted that the term “Common-mode Range”, which defines the
valid range of common-mode voltages, is often used instead of “Voltage Input
Limits.” For DIFF voltages that are small compared to the Input Limits, the
Common-mode Range is essentially equivalent to the Input Limits. Yet as
shown in FIGURE 3.1.1-2, the Common-mode Range =
±⎪Input Limits –
V
dm
/2 ⎪, indicating a reduction in Common-mode Range for increasing DIFF
signal amplitudes. For example, with a 5000 mV DIFF signal, the Common-
mode Range is reduced to
±2.5 V, whereas the voltage Input Limits are
always ±5 V. Hence, the term INPUT LIMITS is used in place of the
widely used term, Common-mode range.
Because a single ended measurement is referenced to CR9000X ground, any
difference in ground potential between the sensor and the CR9000X 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 CR9000X 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 in a
where external signal conditioning circuitry is powered from the same source
as the CR9000X. 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 low input is known to be different from ground, such
as the output from a full bridge.
3.1.2.1 Single Ended Voltage Range
The voltage range for single ended measurements is the range in which the
input voltage must be, relative to CR9000X ground, for the measurement to be
made.
The resolution (the smallest difference that can be detected) for the A/D
conversion is a fixed percentage of the full scale range. To obtain the best
3-5