Campbell Scientific CR9000X Measurement and Control System - Manual (Page 112)

Section 3. CR9000X Measurement Details
• The CR9058E does not directly support Bridge measurements, but Bridge
type measurements can be performed through using the CR9060's CAOs
or external excitation and adjusting the multiplier according to the
excitation level.
3.2.2 CR9058E Sampling, Noise and Filtering
The ten analog to digital converters are re-synchronized at the beginning of
each scan. There is a minimum 1320
microseconds of over-head associated
with this process and other tasks. Therefore the scan, or Subscan, period for
the CR9058E must be greater than 1320 microseconds + the user set
integration time. Since the minimum integration time is 192 (two measurement
samples 96 microseconds apart), the minimum Scan period for the CR9058E is
1520 microseconds. The integration time (microseconds) divided by 96
determines the number of measurements taken during a scan. If reverse
measurement is set true, and/or Open Sense range (V2C) option is selected,
then the over-head will be increased. The CR9058E has a digital signal
processor that performs “Sinc-n” filtering of the analog to digital converter
results to reduce noise. At compile time, unless the Sinc-n filter order is
specified by the user, the CR9058E computes the order of the Sinc-n filter
based on the integration time and Scan interval. The more samples available,
the higher the order of Sinc-n filter is implemented up to an order of five. The
equation used to calculate the filter is:
Eq.3.2.1
( )
( )
SampleTime IntegTime
SampleTime AvailTime
r filterorde
−
−
=
where:
AvailTime = Scan (or Subscan) Interval with the following adjustments:
Subtract off 1520 microseconds if range code v2C is used.
Divide by 2 and subtract off 420 microseconds if input reversal is true.
Subtract off another 1320 microseconds
If resulting AvailTime < 200 microseconds, the user entered scan
interval must be increased.
IntegTime = user entered Integration time in microseconds.
SampleTime = 96 (microseconds)
A first order Sinc filter can be thought of as a simple average of the samples.
The number of values that will be included in the average is dictated by the
integration time (IntegTime/SampleTime). Higher order Sinc filters can be
thought of as running averages feeding running averages. The number of
values used for the running averages at each stage will be the same. Figure
3.2.2-1 is a depiction of a 5th order Sinc filter having a 288 (3 x 96) uSec
integration.
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