Rockwell Automation 1756-OF8I ControlLogix Eight-channel Isolated Analog I/O Modules - Manual (Page 49)

Rockwell Automation Publication 1756-UM540A-EN-P - May 2014 49
1756-IF8I Isolated Analog Input Module Chapter 3
Relationship between Noise Rejection Level and RPI Setting
The 1756-IF8I module offers two levels of line noise rejection. Each level has a
filter associated with it. The module automatically determines which filter is used
based on the notch filter setting and RPI rate.
A trade-off exists between sampling speed and level of noise rejection:
• The faster the sampling speed, the less noise rejection. In this case, the
module automatically uses a SINC^1 filter.
The SINC^1 filter offers 34 dB noise rejection at the notch filter
frequency and its overtones.
• The slower the sampling rate, the better noise rejection. In this case, the
module automatically uses a SINC^3 filter.
The SINC^3 filter offers 100 dB noise rejection at the notch filter
frequency and its overtones.
The following table lists the available notch filter settings, the minimum RPI rate
available with that notch filter setting, and the corresponding noise response.
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)
200.1 ms 100.1 ms 66.7 ms 50.1 ms 20.1 ms 16.7 ms 10.1 ms 2.1 ms 1.1 ms 1.0 ms
Minimum Sample Time
(RPI) - SINC^3 Filter
(1)
600.1 ms 300.1 ms 200.1 ms 150.1 ms 60.1 ms 50.1 ms 30.1 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.