Spectrum Controls 1746sc-INI4vi - Manual (Page 26)

22 SLC 500 ™ Isolated Analog Input Modules
In general, you can reduce the channel update time by doing any of the
following:
• disabling unused channels
• selecting a higher frequency input filter
You can reduce the full-scale step response time by:
• selecting a higher frequency input filter
Note, however, that selecting a higher frequency input filter decreases the
noise rejection and effective resolution, as described in the next
subsection, “Selecting An Input Filter Frequency.”
For the fastest possible channel update time, enable only one channel and
select a 250 or 500 Hz filter. Conversely, your module operates most
slowly if you enable all 4 channels and select a 50 Hz filter for each.
Selecting An Input Filter
Frequency
The 1746sc-INI4i and 1746sc-INI4vi input modules use a digital filter to
reject the high-frequency noise that can couple into an analog input
signal. This digital filter is programmable, letting you select one of four
filter frequencies for each channel. The digital filter provides the highest
noise rejection at the selected filter frequency. For example, to reject the
noise associated with 60 Hz power, you may want to select the 60 Hz
filter.
The filter frequency you select determines that channel’s 3 dB cut-off
frequency, full-scale step response time, worst-case effective resolution,
and channel update time, as shown in Tables 8 and 9.
Table 8. Cut-off frequency, step response time, & effective
resolution (based on filter frequency)
Worst-Case Effective Res. ➀
Filter 3 dB Cut-Off Full-Scale Step Resp. Voltage Mode Current Mode
50 Hz 13.1 Hz 80 ms 12 bits 11 bits
60 15.7 66.7 12 11
250 65.5 16 11.5 10.5
500 131 8 9.5 9.5
➀ For a 16-bit device like the 1746sc-INI4i and -INI4vi, a worst-case effective resolution of
12 bits means that the data may fluctuate by as much as 16 counts (4 bits).
Typically, the effective resolution is greater than that listed here.