Campbell CR6 Series, Руководство оператора

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Section 8. Operation
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referred to as f
N1
. fN1 is a parameter in CRBasic analog measurement
instructions such as VoltDiff() . For example, 50 Hz or 60 Hz noise is common
in areas serviced by 50 Hz or 60 Hz the mains ac power grid. The noise can be
filtered out of the measurement ('rejected') by entering the argument fN1 = 50 or
fN1 = 60 , respectively. The time to complete an A-to-D conversion is
proportional to 1/ fN1 , resulting in a trade-off between noise rejection and
measurement speed. Selecting
fN1
=
60
results in complete rejection of noise at
frequencies of 60 Hz, 120 Hz, 180 Hz, etc. The time to complete the A-to-D
conversion with fN1 = 60 is 1/60, or approximately 17 ms. The overall
measurement time includes 1/ fN1 plus the SettlingTime argument in the same
CRBasic measurement instruction. Excitation reversal or differential-input
reversal doubles this measurement time. Using both in the same measurement
results in four measurements averaged together as the reported value.
All analog function measurements (except vibrating wire) are made at this 93750
samples per second rate. fN1 determines how many of these to average together.
FIGURE 83: Normalized Sinc Frequency Response
Signal Settling Time
Settling time allows an analog voltage signal to settle closer to the true magnitude
prior to measurement. To minimize measurement error, signal settling is needed
when a signal has been affected by one or more of the following:
• A small transient originating from the internal multiplexing that connects
a CR6 terminal with measurement circuitry
•
A relatively large transient induced by an adjacent excitation conductor
on the signal conductor, if present, because of capacitive coupling during
a bridge measurement