Spectrum Controls 1756sc-OF8H Series A - Manual (Page 56)

42 ControlLogix ™ Analog HART Modules
Selecting a low value (i.e. 10 Hz) for the filter frequency provides the best
noise rejection for a channel, but it also increases the channel update time.
Selecting a high value for the filter frequency provides lower noise
rejection, but decreases the channel update time.
The module filter is a built-in feature of the Analog-to-Digital convertor
which attenuates the input signal beginning at the specified frequency.
In addition to frequency rejection, a by-product of the filter selection is the
minimum sample rate (RTS) that is available. For example, the 1000Hz
selection will not attenuate any frequencies less than 1000Hz and will
allow sampling of all 8 channels within 18ms. But the 10Hz selection will
reject all frequencies above 10Hz and will only allow sampling all 8
channels within 488ms. Table 5.2 lists the minimum RTS rate associated
with each ADC filter setting. Refer to table 5.1a to configure this
feature.
Table 5.2
ADC Filter (Hz) Minimum RTS (ms)
10 488
15 328
20 248
50 88
60 88
100 56
250 28
1000 18
Digital Filter Digital Filter Digital Filter Digital Filter Digital Filter
The digital filter smooths input data noise transients on each input channel.
This value specifies the time constant for a digital first order lag filter on
the input. It is specified in units of milliseconds. A value of 0 disables the
filter.
The digital filter equation is a classic first order lag equation.
Yn = Yprevn + (dT/(dT + TA) * (Xn – Yprevn))
Yn = Filtered peak voltage (PV)
Yprevn = Previous Yn
dT = Time expired since previous Yn (Seconds)