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Alias Rejection (DSA, C Series, and NI 433x)

Last Modified: August 10, 2018

DSA devices, C Series Delta-Sigma devices, and NI 433x devices employ a class of ADCs and DACs known as delta-sigma converters. Delta-sigma ADCs include built-in digital filters to provide alias protection from out-of-band signal components. The digital filters always impart a delay of several samples between the time when a given analog voltage level becomes present at the ADC input and when the converter returns the corresponding digitized value. The length of this delay is always deterministic for a particular device running at a given sampling rate.

Likewise, interpolators and delta-sigma DACs provide digital filtering on analog output signals to eliminate out-of-band imaging and quantization noise. As with analog input, the digital output filtering results in a deterministic delay through the DAC.

You can safely ignore the effects of the digital filter delay for most input-only or output-only applications. The filter delay can become significant for applications requiring input and output synchronization such as stimulus-response testing and tight loop control. For DSA and C Series devices, if your application employs external digital triggering, the acquisition returns data that occurred in time before the trigger event. The number of samples preceding the trigger matches the ADC filter delay. Refer to your device documentation for more details on the ADC and DAC digital filter delays.

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