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Why is the NI-5105 Sampling Rate Higher Than its Timebase Frequency?

Hardware: Modular Instruments>>High-Speed Digitizers (Scopes)>>PCI-5105, Modular Instruments>>High-Speed Digitizers (Scopes)>>PXI-5105

Problem:
The specifications for the NI-5105 digitizer list a Frequency Range of 4 Mhz to 65 MHz, but it also states a Timebase Frequency of 60 MHz. Why aren't these two frequency specifications equal?

Solution:
The frequency limit on the ADC used by the NI-5105 is 65 MHz. An external sample clock provided to the board can have any arbitrary frequency, but it cannot exceed 65 MHz.

NI digitizers use a divide-down method of generating internal (onboard) sample clocks. This method divides a timebase clock by an integer value (usually ranging from 1 to 65,535), so that the resulting signal serves as a sample clock of desired frequency. If a 65 MHz timebase were supplied on the board, then the sample clocks available would be odd values, such as:

• 65 MHz
  
• 32.5 MHz
• 21.667 MHz
• 16.25 MHz
• 13 MHz

By providing a 60 MHz timebase instead, the board is able to generate more common sampling frequencies:

• 60 MHz
• 30 MHz
• 20 MHz
• 15 MHz
• 10 MHz

Related Links:
Product Manuals: NI PXI/PCI-5105 Specifications
Product Manuals: NI High-Speed Digitizers Help

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Report Date: 03/08/2007
Last Updated: 12/03/2008
Document ID: 477F54D3

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