Specifications

Setup & SetUp Command 5-10
filter constant after every A/D conversion. The constant selected depends
on the magnitude of the change of the input signal and the setup for the
number of digits displayed. The microprocessor always keeps the value of
the last calculated output to compare to a new data conversion. If the new
data differs from the last output by more than ten counts of the last displayed
digit, the large signal time constant is used in the digital filter. If the result of
the most recent A/D conversion differs from the last output value by less than
ten counts of the last displayed digit, the small signal time constant is used.
Let’s look at an example:
The D1411 RTD module has a standard output resolution of 0.1 degrees.
The standard number-of-displayed-digits setup for this module is 6 digits,
from byte 4 of the setup data. Therefore, the large signal filter will be selected
if a new input conversion differs from the previous value by > 1.0 degree:
Previous data New data Filter selected
+00100.00 +00100.50 small
+00100.00 +00101.50 large
+00100.00 +00099.90 small
+00100.00 +00098.90 large
-00050.50 -00050.00 small
-00050.50 -00060.00 small
If the number of displayed digits is changed to reduce output resolution, filter
selection is also affected. If the number of displayed digits in the previous
example is changed to 5, the output resolution becomes 1.0 degree.
In this case the large signal time constant is used if the new reading differs
from the old by more than 10.0 degrees:
Previous data New data Filter selected
+00100.00 +00105.00 small
+00100.00 +00111.00 large
+00100.00 +00091.00 small
+00100.00 +00085.00 large
-00050.00 -00045.00 small
-00050.00 -00039.00 large
Large Signal Time Constant
The large signal filter time constant is specified by bits 3,4,5 of byte 4. It may
be specified from 0 (no filter) to 16 seconds. The time constant for a first-
order filter is the time required for the output to reach 63% of its final value
for a step input.