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Sample Rate and Decimation

Last Modified: August 8, 2019

The sample rate, specified in samples per second (S/s), is the rate at which a signal is sampled and digitized by an analog-to-digital converter (ADC). According to the Nyquist theorem, a sample rate at least twice the highest frequency of the signal produces accurate measurements if the analog bandwidth is wide enough to let the signal pass through without attenuation. A higher sample rate captures more waveform details for the time domain measurements.

The following figure illustrates a 1 MHz sine wave sampled by a 2 MS/s ADC and a 20 MS/s ADC. The faster ADC digitizes 20 points per cycle of the input signal compared with 2 points per cycle with the slower ADC. In this example, the higher sample rate more accurately captures the waveform shape.

Figure 1. ADC Sample Rate Comparisons

The NI ELVIS III oscilloscope's maximum sampling rate is determined by the speed of the onboard clock, ADC (conversion) rate which is 100 MS/s. However, lower sampling rates can be achieved by decimation, averaging, or storing minimum/maximum samples of the conversions. The valid sampling rates are always an integer divisor of the onboard clock.

Method Description
Decimation In the decimation method, to achieve slower effective sampling rates, the conversions are only stored at specific intervals and the conversions between intervals are discarded. For example, if the onboard clock is 100 MHz, but you want to sample at 25 MS/s, the decimation method discards all conversions except for every fourth conversion to achieve exactly 25% of the maximum real-time sample rate.
Averaging The averaging method calculates and stores the average of multiple ADC samples. For example, if you want to sample at 10 MS/s, the software will return the average of 10 conversions. The averaging also acts as low pass filter, reducing the aliasing effect of frequency components above the specified sample rate.
Storing minimum/maximum samples The minimum/maximum method stores pairs of samples, which are the minimum and maximum conversion values during a sampling interval. Minimum and maximum samples will be displayed interleaved on the same graph. The displayed data rate represents points on the graph per display time interval and because the device returns one minimum and one maximum value for each sample interval, the effective data rate is half of the displayed data rate.

By oversampling, the maximum sample rate can be increased to 400 MS/s. This requires a repetitive signal and a stable trigger configuration. Using this method, multiple consecutive acquisitions are combined in the background, based on the phase of each capture.

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