l Not enough differential terminals are available. Differential measurements use twice as
many analogue input terminals as do single-ended measurements.
l Rapid sampling is required. Single-ended measurement time is about half that of
differential measurement time.
l Sensor is not designed for differential measurements. Some Campbell Scientific sensors are
not designed for differential measurement, but the drawbacks of a single-ended
measurement are usually mitigated by large programmed excitation and/or sensor output
voltages.
Sensors with a high signal-to-noise ratio, such as a relative-humidity sensor with a full-scale
output of 0 to 1000 mV, can normally be measured as single-ended without a significant
reduction in accuracy or precision.
Sensors with a low signal-to-noise ratio, such as thermocouples, should normally be measured
differentially. However, if the measurement to be made does not require high accuracy or
precision, such as thermocouples measuring brush-fire temperatures, which can exceed 2500 °C,
a single-ended measurement may be appropriate. If sensors require differential measurement,
but adequate input terminals are not available, an analogue multiplexer should be acquired to
expand differential input capacity.
Because a single-ended measurement is referenced to datalogger ground, any difference in
ground potential between the sensor and the datalogger will result in an error in the
measurement. For more information on grounds, see Grounds (p. 10) and Minimizing ground
potential differences (p. 49).
6.1.3.2 Minimizing ground potential differences
Low-level, single-ended voltage measurements (<200 mV) are sensitive to ground potential
fluctuation due to changing return currents from 5V,12V, SW12, and C terminals. The datalogger
grounding scheme is designed to minimize these fluctuations by separating signal grounds ( )
from power grounds (G). For more information on datalogger grounds, see Grounds (p. 10). To
take advantage of this design, observe the following rules:
l Connect grounds associated with 5V,12V, SW12, and C terminals to G terminals.
l Connect excitation grounds to the nearest terminal on the same terminal block.
l Connect the low side of single-ended sensors to the nearest terminal on the same
terminal block.
l Connect shield wires to the terminal nearest the terminals to which the sensor signal
wires are connected.
If offset problems occur because of shield or ground wires with large current flow, tying the
problem wires into terminals next to terminals configured for excitation and pulse-count should
6. Measurements 49
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