Campbell Scientific CR800 and CR850 Measurement and Control Systems - Manual (Page 276)

Section 8. Operation
276
8.1.2.5 Voltage Range
In general, a voltage measurement should use the smallest fixed-input range that
will accommodate the full-scale output of the sensor being measured. This results
in the best measurement accuracy and resolution. The CR800 has fixed input
ranges for voltage measurements and an auto range to automatically determine the
appropriate input voltage range for a given measurement. The table Analog
Voltage Input Ranges with CMN / OID
(p. 276)
lists input voltage ranges and range
codes.
8.1.2.5.1 AutoRange
For signals that do not fluctuate too rapidly, range argument AutoRange allows the
CR800 to automatically choose the voltage range to use. AutoRange makes two
measurements. The first measurement determines the range to use, and is made
with the 250-μs integration on the ±5000 mV range. The second measurement is
made using the appropriate range with the integration specified in the instruction.
Both measurements use the settling time programmed in the instruction. Auto-
ranging optimizes resolution but takes longer than a measurement on a fixed
range, because of the two measurements required.
An auto-ranged measurement will return NAN (Not-A-Number) if the voltage
exceeds the range picked by the first measurement. To avoid problems with a
signal on the edge of a range,
AutoRange selects the next larger range when the
signal exceeds 90% of a range.
Auto-ranging is recommended for a signal that occasionally exceeds a particular
range, for example, a Type J thermocouple measuring a temperature usually less
than 476
°C (±25 mV range) but occasionally as high as 500°C (±250 mV range).
AutoRange should not be used for rapidly fluctuating signals, particularly signals
traversing several voltage ranges rapidly. The possibility exists that the signal can
change ranges between the range check and the actual measurement.
Table 52. Analog Voltage Input Ranges with Options for Common
Mode Null (CMN) and Open Input Detect (OID)
Range Code Description
mV5000 measures voltages between ±5000 mV
mV2500 1 measures voltages between ±2500 mV
mV250 2 measures voltages between ±250 mV
mV25 2 measures voltages between ±25 mV
mV7_5 2 measures voltages between ±7.5 mV
mV2_5 2 measures voltages between ±2.5 mV
AutoRange 3 datalogger determines the most suitable range
1 Append with C to enable CMN/OID and set excitation to full-scale (~2700 mV)
2
Append with C to enable CMN/OID
3
Append with C to enable CMN/OID on ranges ≤ ±250 mV, CMN on ranges > ±250 mV