NI 5442
Digital Filter
The main DAC provides an internal digital filter. When digital filtering is enabled, the main DAC runs at a faster rate, referred to as the effective sample rate (ESR). The waveform data transfers to the main DAC at the configured Sample clock rate; the internal digital filter interpolates by a factor of 2x, 4x, or 8x; and the DAC generates the data at the ESR.
Effective sample rate is calculated with the following formula:
ESR = Ifac × SR
where
ESR = the Effective Sample Rate (MS/s)
Ifac = the interpolation factor
SR = configured Sample clock rate (MS/s)
The ESR applies only when the digital filter is enabled. The maximum ESR is 400 MS/s. The ESR is recommended to be as high as possible without exceeding 400 MS/s. The following table lists the allowed update and interpolation rates.
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Note When the OSP block is enabled, you cannot set the Sample Rate property or the NIFGEN_ATTR_ARB_SAMPLE_RATE attribute. After you have configured the IQ Rate, you can read the sample rate. |
| Recommended Interpolation Settings | |
|---|---|
| Sample Clock Rate (MS/s)* | Interpolation |
| 12.5 to 105 | 2x |
| 10 to 100 | 4x |
| 10 to 50 | 8x |
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Notes The digital filter is not available for use for Sample clock rates below 10 MS/s. In Standard Function mode, the digital filter is enabled and disabled by NI-FGEN except when generating user-defined waveforms. |
The delay from the time that the device receives a Start trigger to the time that the analog output signal is generated increases if the digital filter is enabled. The delay increases with an increase in the interpolation factor. Refer to the device specifications for information about the delay from the trigger to the analog output based on the configured filter settings.
In general, use the digital filter for signals containing large sinusoidal waveform content, such as AM and FM, sinc pulse, and sinusoidal chirp waveforms. The filter can be disabled for signals that are better represented without filtering, such as square waves or waveforms containing many pulse characteristics. If you enable the digital filter without setting the interpolation factor, NI-FGEN automatically uses the highest interpolation factor possible in accordance with the preceding table. Refer to the Configure Digital Filter VI or the niFgen_EnableDigitalFilter and niFgen_DisableDigitalFilter functions for more information about setting the digital filter.
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Note NI recommends that you always enable or disable the analog filter (if available) and digital filter at the same time. |