Campbell CR3000 Micrologger Operator'S Manual Download

Page: 339 / 644

Section 8. Operation

339

terminals are not available, an analog multiplexer should be acquired to expand
differential input capacity.

Because a single-ended measurement is referenced to CR3000 ground, any
difference in ground potential between the sensor and the CR3000 will result in an
error in the measurement. For example, if the measuring junction of a copper-
constantan thermocouple being used to measure soil temperature is not insulated,
and the potential of earth ground is 1 mV greater at the sensor than at the point
where the CR3000 is grounded, the measured voltage will be 1 mV greater than
the true thermocouple output, or report a temperature that is approximately 25 °C
too high. A common problem with ground-potential difference occurs in
applications wherein external, signal-conditioning circuitry is powered by the
same source as the CR3000, such as an ac mains power receptacle. Despite being
tied to the same ground, differences in current drain and lead resistance may result
in a different ground potential between the two instruments. So, as a precaution, a
differential measurement should be made on the analog output from an external
signal conditioner; differential measurements MUST be used when the low input
is known to be different from ground.

Integration

The CR3000 incorporates circuitry to perform an analog integration on voltages to
be measured prior to the A-to-D

(p. 529)

conversion. Integrating the the analog

signal removes noise that creates error in the measurement. Slow integration
removes more noise than fast integration. When the duration of the integration
matches the duration of one cycle of ac power mains noise, that noise is filtered
out. The table Analog Measurement Integration

(p. 340)

lists valid integration

duration arguments.

Faster integration may be preferred to achieve the following objectives:

•

Minimize time skew between successive measurements

•

Maximize throughput rate

•

Maximize life of the CR3000 power supply

•

Minimize polarization of polar sensors such as those for measuring
conductivity, soil moisture, or leaf wetness. Polarization may cause
measurement errors or sensor degradation.

•

Improve accuracy of an LVDT measurement. The induced voltage in an
LVDT decays with time as current in the primary coil shifts from the
inductor to the series resistance; a long integration may result in most of
signal decaying before the measurement is complete.

Note See White Paper "Preventing and Attacking Measurement Noise
Problems" at www.campbellsci.com.

The magnitude of the frequency response of an analog integrator is a SIN(x)/x
shape, which has notches (transmission zeros) occurring at 1/(integer multiples) of
the integration duration. Consequently, noise at 1/(integer multiples) of the
integration duration is effectively rejected by an analog integrator. If reversing the

Campbell CR3000 Micrologger, Operator'S Manual

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