Rockwell Automation 1746-INT4 Thermocouple/MV Isolated/ User Manual - Manual (Page 38)

Publication 1746-UM614B-EN-P - September 2007
38 Preliminary Operating Considerations
Input Channel
Characteristics Each channel has an 8 Hz digital filter for input noise rejection, a
multiplexer for processing cold junction compensation (CJC) values,
and an analog-to-digital (A/D) converter to provide digital values for
SLC processing.
Channel Cut-off Frequency, Update Time, and Step Response
The channel cut-off frequency is defined as the point on the
frequency response curve where frequency components of the input
signal are passed with 3 dB of attenuation by the input filter. All
frequency components above cut-off frequency are increasingly
attenuated. Cut-off frequency is also defined as the Normal Mode
Rejection (NMR) in dB of attenuation at 50 Hz (European) or at 60 Hz
(American).
See the Cut-off Frequencies graph on page 39.
We define module update time as the time required for the module to
sample and convert channel input signals, multiplex them with the
CJC reference value, and make the resulting values available to the
SLC processor. It is typically 200 ms for multiplexing and 200 ms for
sampling and converting.
When sampling occurs after the signal reaches 99.9% of final value,
the update time defines the minimum time (400 ms) for processing an
input signal.
When sampling occurs just before the signal reaches 99.9% of final
value, we define step response (worst-case) as the sum of the times
required for the analog input signal to change from 0…99.9% of its
expected final value (see graph). It includes the times required for:
•
input filter: 180 ms.
•
CJC multiplexer: 200 ms.
•
A/D converter: 200 ms.
This defines the maximum time required for processing an input
signal.