Cut-Off Frequency; Repeatability; Module Update Time - Spectrum Controls 1769 User Manual

Chapter 3: Module, Data, Status, and Channel Configuration
common mode voltage rating (±10V) of the module. Improper earth ground may
be a source of common mode noise.
Transducer power supply noise, transducer circuit noise, or process
NOTE
variable irregularities may also be sources of normal mode noise.

Cut-Off Frequency

The filter cut-off frequency, -3 dB, is the point on the frequency response curve
where frequency components of the input signal are passed with 3 dB of
attenuation. Table 3-6 shows cut-off frequencies for the supported filters.
All input frequency components at or below the cut-off frequency are passed by
the digital filter with less than 3 dB of attenuation. All frequency components
above the cut-off frequency are increasingly attenuated.
The cut-off frequency for each channel is defined by its filter frequency
selection. Choose a filter frequency so that your fastest changing signal is below
that of the filter's cut-off frequency. The cut-off frequency should not be
confused with the update time. The cut-off frequency relates to how the digital
filter attenuates frequency components of the input signal. The update time
defines the rate at which an input channel is scanned and its channel data word is
updated.

Repeatability

Repeatability is the ability of the input module to register the same reading in
successive measurements for the same input signal. The repeatability for an
input channel depends upon the filter frequency selected for that channel. Table
3-5, above, describes the repeatability for each of the range selections at the six
available frequencies. This table does not include the effects of unfiltered input
noise. Choose the frequency that most closely matches your requirements.

Module Update Time

The module update time is defined as the total time required for the module to
sample and convert the input signals of all enabled input channels and provide
the resulting data values to the processor. On an isolated module with one ADC
per channel, all channels convert the signal in parallel so the module update time
is equal to the slowest channel update time. When lead compensation is enabled
(default setting) one out of six channels in each module scan has both the
RTD/resistance voltage measured AND the lead compensation voltage measured.
This repeats in a round robin fashion cycling through all enabled channels. An
internal multiplexor in the ADC switches between measuring the lead voltage
and RTD/resistance voltage. This switching increases the settling time for each
measurement and the overall channel update rate increases to the slower update
rates listed in Table 3-5.
Channel update time is also dependent upon the input filter selection. Table 3-6
shows the channel update times based on filter selection.
User's Manual Pub. 0300241-02 Rev. A
3-9