Campbell Scientific CR9000X Measurement and Control System - Manual (Page 108)

Section 3. CR9000X Measurement Details
3.1.6 Measurements Requiring AC Excitation
Some resistive sensors require AC excitation. These include electrolytic tilt
sensors, soil moisture blocks, water conductivity sensors and wetness sensing
grids. The use of DC excitation with these sensors can result in polarization,
which will cause an erroneous measurement, and may shift the calibration of
the sensor and/or lead to its rapid decay.
Other sensors like LVDTs (without built in electronics) require an AC
excitation because they rely on inductive coupling to provide a signal. DC
excitation would provide no output.
Any of the bridge measurements can reverse excitation polarity to provide AC
excitation and avoid ion polarization. The frequency of the excitation can be
determined by the delay and integration time used with the measurement. The
highest frequency possible is 50 kHz, the excitation is switched on and then
reversed 10 µs later when the first measurement is held and then is switched
off after another 10 µs when the second measurement is held (i.e., reverse the
excitation, 10 µs delay, no integration).
A switched excitation channel (7-16 on the CR9060 Module)
should be used when AC excitation is required because it will be
switched out as soon as the measurement is completed. The
continuous excitation channels (1-6 on the CR9060 Module)
should not be used because they retain the last voltage
programmed (i.e., after reversing the excitation, the channel
would be left at the reversed polarity voltage until the next
instruction that acted on the excitation channel). TIP
3.1.7 Influence of Ground Loop on Measurements
When measuring soil moisture blocks or water conductivity the potential exists
for a ground loop which can adversely affect the measurement. This ground
loop arises because the soil and water provide an alternate path for the
excitation to return to CR9000X ground, and can be represented by the model
diagrammed in Figure 3.1.7-1.
FIGURE 3.1.7-1. Model of resistive sensor with ground loop
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