Sample Rate

NI Digital Multimeters Help (NI-DMM 18.1)

Edition Date: March 2018

Part Number: 370384V-01

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Sample rate is the rate at which a signal is sampled by an ADC. A higher sample rate captures more waveform details while a sample rate that is too low can distort the waveform.

The available sample rates (r) correspond to the following equation:

r = (1.8 MS/s)/y

where y = 1, 2, 3,...1.8 x 105.

The figure below illustrates a 100 kHz sine wave sampled at 500 kS/s:

The figure below shows the same sine wave sampled at 1.8 MS/s:

The faster rate digitizes 18 points per cycle of the input signal compared with 5 points per cycle with the slower ADC. In this example, the higher sample rate more accurately captures the waveform shape as well as frequency. This process is called over-sampling and can greatly improve the quality of the waveform measurement. The DMM allows for 6 times over-sampling when acquiring a voltage waveform at the limit of its analog bandwidth of 300 kHz.

When performing a waveform acquisition, select a sampling rate that observes the Nyquist Theorem. Excessive over-sampling has tradeoffs. First, excessive over-sampling may give you more data than you need, unnecessarily consume processor resources, and slow the display. Second, additional noise becomes apparent, because the noise performance of the DMM improves as your sampling rate decreases.

Note Note  The NI 4080/4081/4082 does not support the use of DC Noise Rejection in waveform mode.


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