Spectrum Controls 1769sc-IF8u - Manual (Page 56)

46 Compact IO ™ Universal Input module
Selecting Input Filter Frequency (Bits 3 through 1)
The input filter selection field allows you to select the filter frequency for
each channel and provides system status of the input filter setting for
channels 0 through 7. The filter frequency affects the following, as
explained later in this chapter:
noise rejection characteristics for module inputs
channel step response
channel cut-off frequency
effective resolution
module update time
Effects of Filter Frequency on Noise Rejection
The filter frequency that you choose for a module channel determines the
amount of noise rejection for the inputs. A lower frequency (50 Hz versus
500 Hz) provides better noise rejection and increases effective resolution,
but also increases channel update time. A higher filter frequency provides
lower noise rejection, but decreases the channel update time and effective
resolution.
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 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.
Improper earth ground may be a source of common mode noise.
NOTE: Transducer power supply noise, transducer circuit noise, or
process variable irregularities may also be sources of normal mode noise.
NOTE: 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.
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