DSA(Dynamic Signal Acquisition) devices use a special class of converters, both analog-to-digital (ADC) and digital-to-analog (DAC), known as Delta-Sigma or Sigma-Delta Converters. Many of the features that make DSA devices well-suited for frequency domain measurements are directly related to the use of delta-sigma parts. These features include excellent linearity, dynamic range, and transparent digital anti-aliasing filters.
Unlike the converters found in traditional multifunction DAQ devices, delta-sigma converters require a special timing signal known as the oversample clock. This clock is so named because it runs a frequency many times greater than the desired sampling rate.
On some devices, such as the NI 4472 and NI 4461/4462, the oversample clock rate is based on the requested sampling rate. Other devices, such as the NI 4464 and NI 4480/4481, have a constant oversample clock rate regardless of the requested sampling rate, and is equivalent to the modulator sample rate provided in specifications documents.
As an example of a device with a variable oversample clock, on the NI 4472 the oversample clock runs at exactly 128 times the desired sampling rate when the sampling rate is 51.2 kS/s or less. At rates greater than 51.2 kS/s, the oversample clock runs at 64 times the sampling rate. Thus, at 51.2 kS/s or 102.4 kS/s, the oversample frequency is 6.5536 MHz (51.2k * 128 = 6.5536M, 102.4k * 64 = 6.5536M). These rates dictate some fundamental characteristics of the digital filters, as described in the NI 447x Specifications, look at the figure below.
For more information about Delta-Sigma ADCs and oversampling, please see the Benefits of Delta-Sigma Analog-to-Digital Conversion