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Table Of Contents

Reducing Signal Noise

Last Modified: March 11, 2020

Configure hardware and software filters in FlexLogger to reduce noise in your signals.

FlexLogger supports both hardware and software filters for use in your project. Hardware filters offer a variety of filter type options to provide anti-aliasing and noise reduction with no CPU usage or reduction in system performance but are only available on specific modules. Software filters can be used on all input channels of any hardware supported by FlexLogger, but require high CPU resource usage and also have higher latency.

Using a Hardware Filter

FlexLogger applies the following hardware filters on supported devices. Refer to the Supported Hardware topic for information. Filter types vary depending on the module being used.

  1. Hover over the channel row to see the Configure gear . Click the gear for the channel you wish to configure the filter for.
  2. From the Channel Configuration dialog, click the Filtering tab and select from the following filter types.
    Table 1. Lowpass Filters
    Filter Type Description Comments
    Brickwall Best anti-aliasing Filter is automatically configured to a cutoff frequency of half the data rate.
    Butterworth Better noise rejection, configurable filter order (2nd or 4th order)

    Flexlogger selects the default cutoff frequency based on the timing configuration and selected cutoff band; changes made to the cutoff settings are applied to all module channels and may affect other timing configurations on the module.

    Available on FieldDAQ devices.

    Better noise rejection

    FlexLogger uses the highest cutoff frequency as the default.

    Available on the NI 9252/9253 and PXIe-4300/4310.

    Elliptical Better noise rejection

    FlexLogger uses the highest cutoff frequency as the default.

    Available on the PXIe-4302/4303/4304/4305.

    Digital Glitch Configurable input filter

    Pulses shorter than the specified Pulse Width Cutoff are rejected.

    Available on the NI 9361.

    Note: For C Series and FieldDAQ, any changes made to the filter configuration settings (if available) are applied to all module channels.

    The lowpass filter is applied when the Filter Type is selected, or when the filter configuration is set or changed (if configuration options are available for the selected filter).

    Table 2. ADC Timing Mode-Based Filters
    Filter Mode Description
    Auto FlexLogger uses the highest resolution filter that is valid for the configured data rate.
    None (High Speed) Best sample rate and signal bandwidth, lower accuracy and noise rejection; this is the default filter applied to the channel.
    High Resolution Best accuracy and noise rejection, rejects power line frequencies, higher conversion time and lower signal bandwidth.
    Medium Resolution Better accuracy and noise rejection, rejects power line frequencies, higher conversion time and lower signal bandwidth.
    Medium Speed Better sample rate and signal bandwidth, lower accuracy and noise rejection.
    Variable Timing Modes

    Selectable timing modes ranging from High Resolution to High Speed, allowing for incremental selection of noise and accuracy ratios.

    Available on the PXIe-4353/PXIe-4357.

    • (PXIe-4353) —Timing mode 1 (High Resolution) to timing mode 7 (High Speed).
    • (PXIe-4357) —Timing mode 1 (High Resolution) to timing mode 9 (High Speed).

    Refer to the hardware documentation for additional ADC timing mode information on these modules, such as corresponding data rates and ADC conversion time.

    Best 50 Hz Rejection Optimizes 50 Hz noise rejection.
    Best 60 Hz Rejection Optimizes 60 Hz noise rejection.

The ADC timing mode filter is applied when the Filter Mode is selected, or when the filter configuration is set or changed (if configuration options are available for the selected filter).

Using a Software Filter

Refer to the Configuring Lowpass/Highpass Filter Channels topic for information on creating and configuring a software-based Butterworth filter channel.


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