XLINK 500/100 Operations & Maintenance Manual page # 85
Input Range
This setting is relevant only to analog differential measurements. Remember that the system
must read a voltage that falls into the common mode range mentioned in the above note.
Voltages that are negative with respect to the analog ground terminal may not be measured.
The following options are available:
-39 to +39mV
-312 to +312mV
-2.5 to +2.5V
-39 to +39mV with Bias
-312 to +312mV with Bias
Note that the ranges for absolute measurements are nominal. Guaranteed analog input
range over temperature is 0-4.98 V, ± 2.49 V, ± 311 mV, and ± 38.9 mV.
Choose the option that is close to and greater than the input range of the sensor that is being
connected.
For example, if a sensor provides a reading from 0 to 100mV, choose the 312mV option. If you
were to choose the 39mV option, when the sensor provided a reading greater than 39mV, the
unit would indicate a sensor failure.
Use the range “with Bias” if the sensor is not powered by the station. Pyranometer and
thermocouple type sensors should use bias.
4-20 mA
Connection: 4-20ma (Terminal #20)
This input is designed to function with sensors that have a 4 to 20ma current loop interface. This
type of interface is superior to voltage outputs when the cables to the sensors must travel a long
distance or when the equipment is located in electrically noisy environments. The station will
measure the current flowing when connected to the 4-20ma Input. Typical 4-20ma sensors will
give a 4 to 20ma current for a 0 and 100% FS. Readings that are greater than 21mA will be
considered a fault. The current required for the sensor is provided by the 2 wire loop and does
not typically require additional connections.
NOTE: The station does not provide a dedicated power supply for 4-20ma sensors. The sensor
shall have its own supply or run off the 12 volt supply of the station. While the internal 4-20
measurement is made using an accurate 100 Ohm resistor, the overall loop resistance is 200
ohms due to an additional 100 Ohms series protection resistor. This means that a sensor at a full
20 ma current output, approximately 4 volts should be allotted for overhead voltage. This is
typically not an issue when an external voltage source is provided to operate the sensor that may
be in the 18 to 24-volt range. However, if a 12.5-volt battery is used to source the voltage for the
sensor in the current loop, then only 8.5 volts will be supplied to the sensor at max sensor
current output. Therefore if a full 12 volts is necessary for the sensor, an external loop supply
will be necessary or alternately the sensor may be wired to a single-ended voltage input with the
use of an external 100 Ohm accurate and stable resistor also attached to the voltage input port.
Warmup
Analog sensors are powered by the station via one of several outputs:
VREF (reference voltage) which provides 2.5V, terminals #25 and #21.
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