Campbell Scientific CR9000X Measurement and Control System User Manual
Page 117
Section 3. CR9000X Measurement Details
Due to the minimized signal attenuation at lower frequencies,
and the improved filtering of the higher order frequencies, when
manually selecting the Sinc filter order, we recommend using the
Fifth Order for most applications.
TIP
The actual method used for determining integration time follows:
1. First we determine the time available (AvailTime) for measurement
integration/filtering.
AvailTime = Scan or Subscan Interval (micro-seconds) 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 < (FilterOrder +1) * 96 microseconds, the
user entered scan interval must be increased.
2. Next we calculate N, the number of 96 micro-second (CR9058 base
sample time) integrated values that will be averaged together before the
Sinc-n filter is applied. The CR9058 firmware limits the Number of
Samples that are feed to the filter to 680. This is done to reduce the
amount of time required to compute the weighting coefficients for the
samples that are fed to the Sinc-n filter.
As shown previously, when setting the integration time, the filter order
would be incrementally reduced to limit the number of samples to 680
(covered in section 3.2.2). In the case where the Filter order is hard set,
another method is used to constrain the number of samples: Groups of
samples may be pre-averaged, so that no more than 680 samples go to the
filter, and yet we can integrate over the full available time.
Equation 3.2.5 is used to calculate N, the number of pre-averages.
1
680
/
+
⎟⎟
⎠
⎞
1
96
⎜⎜
⎝
⎛
⎟
⎠
⎞
⎜
⎝
⎛
−
=
uSec
AvailTime
Integer
N
Eq 3.2.5
3. Using the calculated available time (AvailTime) and number of pre-averages
(N) along with the Filter Order, the integration time can be calculated:
(
)
N
uSec
r
FilterOrde
N
uSec
AvailTime
Integer
IntegTime
Ч
Ч
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⎟
⎠
⎞
⎜
⎝
⎛
−
Ч
=
96
1
/
1
96
Eq 3.2.6
3-29