Campbell CR6 Series, Operator'S Manual

Page: 421 / 690

Section 8. Operation
421
Parameters that Control Measurement Sequence and
Timing
CRBasic Instruction Parameter Action
MeasOfs
Correct ground offset on single-ended
measurements.
SettlingTime Sensor input settling time.
f
N1
First notch frequency
RevDiff
Reverse high and low differential
inputs.
RevEx Reverse polarity of excitation voltage.
First Notch Frequency (fN1) Digital Filter
The A-to-D conversion is accompanied by digital filtering, which serves two
purposes:
•
Arrive as close as possible to the true input signal
• Filter out measurement noise at specific frequencies, the most common
being noise at 50 Hz or 60 Hz, which originate from mains-power lines.
Filtering time is inversely proportional to the frequency being filtered.
Minimizing filtering time may be preferred at times to accomplish the following:
• minimize time skew between successive measurements.
•
maximize throughput rate.
•
maximize life of the CR6 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 filtering time may result in most
of signal decaying before the measurement is complete.
Another way to think of the f notch is that 1/fN1 is the integration time. The
shorter the integration time (the larger the f notch), the less filtering or smoothing
of the signal.