Rockwell Automation Publication 1756-UM540E-EN-P - December 2017 45
1756-IF8I Isolated Analog Input Module Chapter 3
1756-IF8I Notch Filter Setting
The following table lists the available Notch Filter settings, the minimum RPI
rate available with that Notch Filter setting, and the corresponding noise
response.
For example, if your application requires a Notch Filter setting of 50 Hz, the
module’s minimum RPI rate is 20.7 ms. In this case, sampling speed is more
important than noise rejection. The module automatically uses a SINC^1 filter.
If your application requires a Notch Filter setting of 50 Hz and the greater level of
noise rejection provided by a SINC^3 filter, the minimum RPI rate is 62.1 ms.
The module automatically uses a SINC^3 filter.
The RPI must be > 1/Notch Filter plus some small scan time for the ADC to
sample properly. The SINC^3 filter takes three times as long and thus requires
RPI > 3/Notch plus some small scan time. The module rejects combinations
which violate that relationship. Table 9
lists the available Notch Filter settings
and the RPI values for the two types of filters.
For more information on Notch Filter settings, see page 131
.
Table 8 - 1756-IF8I Notch Filter Settings and the Minimum RPI Rate
Notch Filter Setting 5 Hz 10 Hz 15 Hz 20 Hz 50 Hz 60 Hz
(Default)
100 Hz 500 Hz 1000 Hz 5000 Hz
Minimum Sample Time
(RPI) - SINC^1 Filter
(1)
207.0 ms 103.5 ms 69.1 ms 51.8 ms 20.7 ms 17.3 ms 10.4 ms 2.1 ms 1.1 ms 1.0 ms
Minimum Sample Time
(RPI) - SINC^3 Filter
(1)
621.0 ms 310.5 ms 207.1 ms 153.4 ms 62.1 ms 51.9 ms 31.2 ms 6.1 ms 3.1 ms 1.0 ms
0…100% Step Response
Time
(2)(3)
600 ms +
1RPI
300 ms +
1RPI
200 ms +
1RPI
150 ms +
1RPI
60 ms +
1RPI
50 ms +
1RPI
30 ms +
1RPI
6 ms + 1 RPI 3 ms +
1RPI
(4)
1 ms +
1RPI
(4)
-3 dB Frequency
(2)
1.3 Hz 2.7 Hz 4.3 Hz 5.1 Hz 13 Hz 15 Hz 26 Hz 128 Hz 258 Hz 1296 Hz
Typical Effective
Resolution
(2)
21 bits 20 bits 20 bits 20 bits 20 bits 20 bits 19 bits 18 bits 18 bits 17 bits
(1) The minimum RPI value for the module depends on the channel with the lowest Notch Filter setting. For example, if three of the channels on a module use a Notch Filter setting of 20 Hz and one channel
uses a Notch Filter setting of 60 Hz, you cannot set the module RPI lower than 50.1 ms.
(2) Using the SINC^3 filter.
(3) Worst case settling time to 100% of step change includes 0…100% step response time plus one RPI sample time.
(4) Value represents module performance in Current mode. For the value when the module is used in Voltage mode, include additional 3 ms settling time due to RC time constant of 7500 Ω voltage input
resistor.
Table 9 - 1756-IF8I Notch Filter Settings and the RPI Values
Notch Filter Fastest RPI for a SINC^1 Filter Fastest RPI for a SINC^3 Filter
5 Hz 207.0 ms 621.0 ms
10 Hz 103.5 ms 310.5 ms
15 Hz 69.1 ms 207.1 ms
20 Hz 51.8 ms 153.4 ms
50 Hz 20.7 ms 62.1 ms
60 Hz (default) 17.3 ms 51.9 ms
100 Hz 10.4 ms 31.2 ms
500 Hz 2.1 ms 6.1 ms
1000 Hz 1.1 ms 3.1 ms
5000 Hz 1.0 ms 1.0 ms
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