Rockwell Automation 1746-NI16V SLC 500 Analog Input Modules User Manual - Manual (Page 46)

Publication 1746-UM001A-US-P
4-10 Preliminary Operating Considerations
Aliasing Frequency
Aliasing is a natural characteristic of discrete time sampling of analog
signals. This can result in erroneous data in the data channel word.
Aliasing is usually not a problem because the duration of the
high-frequency signal is much shorter than the program scan time.
Aliasing begins at a lower frequency when more channels are
enabled. Anti-aliasing filters are available, but you should check your
transducer for malfunctions first, as the filters are fairly expensive.
Aliasing occurs when unwanted signals, at a frequency greater than
the effective sampling rate, are present. The effective sampling rate is
determined by the number of enabled channels (n). The first aliasing
frequency occurs at: f
a
= (1 / Update Time in seconds). Subsequent
aliasing frequencies occur at integer multiples of f
a
.
Noise Rejection
Rejection of common mode noise is inherent in the hardware design
of the module. Common mode rejection is better than -100 dB for
common mode 50 Hz and 60 Hz AC signals. The module performs
well in the presence of common mode noise as long as the signals
applied to the user terminals do not exceed the common mode
voltage rating (±10.25 Volts) of the module. Improper earth/chassis
ground connections may be a source of common mode noise.
Rejection of normal mode noise is implemented in the firmware and is
a function of the filter frequency selected by the user. A lower
frequency filter rejects more normal mode noise than a higher
frequency filter. Transducer power supply noise, transducer circuit
noise, or process variable irregularities may be sources of normal
mode noise.