Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 41)

Publication 1762-UM002A-EN-P - July 2002
Module Data, Status, and Channel Configuration 3-11
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 10Hz 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.
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
Improper earth ground may be a source of common
mode noise.
Transducer power supply noise, transducer circuit
noise, or process variable irregularities may also be
sources of normal mode noise.
TIP
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.
Filter Frequency Step Response
10 Hz 303 ms
50 Hz 63 ms
60 Hz 53 ms
250 Hz 15 ms
500 Hz 9 ms
1 kHz 7 ms