Notch filter

The notch filter is a built-in feature of the Analog-to-Digital converter, which attenuates the input signal at the specified frequency and each of its harmonics. Choose a notch filter based on what noise frequencies are present in the device’s operating environment versus any speed requirements needed for control. The graph below is an illustration of a 10 Hz notch filter.

For example, typically the notch filter value of 60 Hz is used in the United States to filter out 60 Hz AC line noise that might be present on the factory floor. A 60 Hz notch filter setting attenuates frequencies of 60 Hz (60 Hz, 120 Hz, 180 Hz...). A speed trade-off does occur with a 60 Hz notch filter setting in that the fastest sample rate (RTS) achievable by ControlLogix isolated analog devices is limited to 16.7 ms.

There is a relationship between the notch filter and sampling setting provided by the device's RPI.

The device also offers two levels of notch filter settings, one offering more speed, but less noise rejection, and the other superior noise rejection in the form of a SINC^3 filter. The fast filter setting offers 34 dB noise rejection at the notch filter frequency and its overtones while the SINC^3 filter provides 100 dB noise rejection. As a result, the device automatically selects a SINC^3 filter if the notch filter and RPI relationship allows it.

The RPI must be > 1/Notch Filter plus some small scan time for the ADC to sample properly. The SINC^3 filter takes 3 times as long and thus requires RPI > 3/Notch plus some small scan time. The device rejects combinations that violate that relationship.

This table lists the available notch filter settings and the RPI values for the two types of filters.