Campbell Scientific CR9000X Measurement and Control System - Manual (Page 94)

Section 3. CR9000X Measurement Details
resolution, select the smallest range that will cover the voltage output by the
sensor. For example, the resolution of an A/D conversion made on the ± 50
mV range is 1.6
μV; the resolution on the ±5000 mV range is 160 μV. A
copper-constantan thermocouple outputs a voltage of about 40 μV / °C
(difference in temperature between the measurement and reference junction).
The temperature resolution on the ± 50 mV range is 0.04 degrees (1.6
μV /
40 μV / 1°C); the resolution on the ±5000 mV range is 4 degrees (160 μV /
40 μV / °C). Because the smallest ± 50 mV range will allow a 1250 degree
difference (0.05 V / 0.00006 V), which is greater than the sensor capability
(-200 to 400 degrees C) there is no reason to use a larger range.
3.1.2.2 Differential Voltage Range
When a differential voltage measurement is made, the high (H) input is
referenced to the low (L) input. To obtain the best resolution, select the
smallest range that will cover the voltage output by the sensor as described for
single ended voltage measurements above.
Range Code C option: Open Sensor Detect
Sensors that have a floating output (the output is not referenced to ground
through a separate connection, such as thermocouples) may float outside of the
Input Limits, causing measurement problems. For example, a larger static
charge in Example 3.1.2-1 could result in an invalid thermocouple
measurement. Hence, the ability to null any residual common-mode voltage
prior to measurement is useful in order to pull the H and L Instrumentation
Amp inputs within the
±5 V Input Limits. Adding a “C” to the end of the
range code (i.e. mV50C) enables the nulling of the common-mode voltage
prior to a differential measurement for the
±50 mV and ±200 mV input ranges.
The “C” range code option results in a brief internal connection of the H and L
inputs of the IA to 2800 mV and ground, respectively, while still connected to
the sensor to be measured. The resulting internal common-mode voltage is
≈
1400 mV, which is well within the ±5 V Input Limits. Upon disconnecting the
internal 2800 mV and ground connections, the associated input is allowed to
settle to the desired sensor voltage and the voltage measurement is made. If the
associated input is open (floating), the input voltages will remain near the 2800
mV and ground, resulting in an over range (NAN) on the
±50 mV and ±200
mV input ranges. If the associated sensor is connected and functioning
properly, a valid measured voltage will result. When this option is selected,
the time required for each measurement will be increased by 10 micro-seconds.
Example 3.1.2-2: Start with example 3.1.2-1. If the static
charge were to build up to 5000 mV, with a thermoelectric voltage
of 10 mV the V
H
would equal 5005 mV. This is above the Input
Limit of 5000 mV, and a reliable measurement cannot be made on
the CR9050 or CR9051E modules without pulling the inputs to
within the allowable Input Limit range. If the 50mVC, Open Sense
Detect, range code, were utilized, the input voltages would be
pulled within the Input Levels and a good measurement could be
made.
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