Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 47)

Rockwell Automation Publication 1769-UM004B-EN-P - March 2010 47
Module Data, Status, and Channel Configuration Chapter 4
When selecting a filter frequency, be sure to consider cut-off frequency and
channel step response to obtain acceptable noise rejection. Choose a filter
frequency so that your fastest-changing signal is below that of the filter’s cut-off
frequency.
Common Mode Rejection is better than 115 dB at 50 and 60 Hz, with the 50 and
60 Hz filters selected, respectively, or with the 10 Hz filter selected. The module
performs well in the presence of common mode noise as long as the signals
applied to the user positive and negative input terminals do not exceed the
common mode voltage rating (±10V) of the module. Improper earth ground may
be a source of common mode noise.
Effects of Filter Frequency on Channel Step Response
The selected channel filter frequency determines the channel’s step response. The
step response is the time required for the analog input signal to reach 100% of its
expected final value, given a full-scale step change in the input signal. This means
that if an input signal changes faster than the channel step response, a portion of
that signal will be attenuated by the channel filter. The channel step response is
calculated by a settling time of 3 x (1/filter frequency).
TIP Transducer power supply noise, transducer circuit noise, or process
variable irregularities may also be sources of normal mode noise.
The filter frequency of the module’s CJC sensors is the lowest filter
frequency of any enabled thermocouple type to maximize the trade-offs
between effective resolution and channel update time.
Table 4 - Filter Frequency and Step Response
Filter Frequency Step Response
10 Hz 300 ms
50 Hz 60 ms
60 Hz 50 ms
250 Hz 12 ms
500 Hz 6 ms
1 kHz 3 ms