What is the DSA Oversample Clock and What is Its Frequency?Hardware: Modular Instruments>>Dynamic Signal Acquisition and Analysis (DSA)
Problem: I have seen one or more references to the "oversample clock" or "master clock" for Dynamic Signal Acquisition (DSA) devices. These references include the NI 447x Specifications, documentation for the NI PXI-6653 timing and synchronization module, as well as several example programs from the Developer Zone that address questions of synchronization. What is this oversample clock, why is it important, and what is its frequency? Solution: DSA devices use a special class of converters, both analog-to-digital (ADC) and digital-to-analog (DAC), known as "Delta-Sigma 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 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. These rates dictate some fundamental characteristics of the digital filters, as described in the NI 447x Specifications. Sharing the oversample clock is an essential step in obtaining synchronization with very little phase difference between two or more DSA devices. However, the actual clock signal that is shared by both the PCI and PXI versions of the 4472 is actually twice the oversample rate. Thus, the clock on the PXI_Star bus in a synchronized 4472 acquisition may be as high as 13.112 MHz. If you import this signal from a counter, for instance, you must then divide it by either 128 (Fs > 51.2 kS/s) or 256 (Fs = or < 51.2 kS/s) to obtain the DSA device sample rate. Related Links: Product Manuals: NI Dynamic Signal Acquisition User Manual (August 2008) Product Manuals: NI 446x Specifications (December 2008) Product Manuals: NI 447x Specifications (March 2006) Product Manuals: NI 449x Specifications (July 2008) Product Manuals: NI USB-9233 User Guide and Specifications (December 2007) Attachments:
Report Date: 08/28/2003 Last Updated: 03/24/2009 Document ID: 30RIMC2L |
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