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Data Acquisition Sampling Terminology

Hardware: Multifunction DAQ (MIO)

Problem:
I am confused about some of the vocabulary used to describe the data acquisition process. How are the terms "sample clock," "sample rate," "convert clock," "convert rate," "channel list," "scan," "interval scanning," and "interchannel delay" defined?

Solution:
NI-DAQ 7.0 contains three separate drivers—NI-Switch, Traditional NI-DAQ, and NI-DAQmx. NI-DAQmx introduced a new data acquisition (DAQ) architecture, including terminology changes. These terminology changes improve consistency across the LabVIEW and C APIs. The table below demonstrates this, translating terms from Traditional NI-DAQ to NI-DAQmx for both the C API and LabVIEW API. To view the complete translation table, please refer to the Traditional NI-DAQ versus NI-DAQmx listing in the NI-DAQmx Help (Start » Programs »National Instruments » NI-DAQ » NI-DAQmx Help) under the Contents tab.

The terms defined in the table below are commonly used when discussing sampling. These terms should be understood before moving on to the definitions listed below the table. Please note that in the definitions after the table, NI-DAQmx terminology is used by default; if there is an equivalent Traditional NI-DAQ term it is included in parenthesis.

Traditional NI-DAQ C API Traditional NI-DAQ LabVIEW NI-DAQmx Explanation
Scan Interval Counter Scan Clock Sample Clock The clock that controls the time interval between samples. Each time the sample clock ticks (produces a pulse), one sample per channel is acquired. The source of the sample clock may be regulated by an internal timer on the data acquisition (DAQ) board or by an external clock signal.
Scans per Second Scans per Second Samples per Channel per Second Units for specifying sample acquisition rate (the sample rate).
Sample Interval Counter Channel Clock or Interchannel Delay Convert Clock The E Series clock that directly causes analog-to-digital conversions. By default, the convert clock rate is calculated automatically by the NI-DAQ driver software. You can also specify the convert clock rate explicitly by setting the convert clock to be based on an internal timer on the data acquisition board or an external clock signal.
Points per Second Channels per Second Conversions per Second Units for specifying convert clock rate.

Notes:
  • Some non-plug and play (legacy) DAQ boards do not have a sample (scan) clock—they only have a convert (channel) clock. These boards use round-robin sampling (see the related link below).
  • Simultaneous sampling (S-series) boards have only a sample (scan) clock. Since these boards are not multiplexed, they have no need for a convert (channel) clock.

Channel list (scan list)
List of channels to be sampled.

Scan
A set of samples, one from each channel in the channel list.

Interval scanning
A multi-channel sampling method. This method uses the sample (scan) clock to control when a scan begins and uses the convert (channel) clock to control when each channel in the channel list is sampled.

Sample/Sampling rate (scan rate)
The rate at which one sample per channel is acquired; the rate at which the sample (scan) clock is set. Note that the maximum sampling rate specification for our E Series data acquisition boards refers to the maximum rate at which a single channel can be acquired. For example, the PCI-6071E is capable of a 1.25 MS/s sampling rate. This may consist of 1.25 MS/s on one channel or 250 kS/s on five channels.
Units: Samples per Channel per Second (Scans per Second)

Convert rate (channel rate)
The rate at which the analog-to-digital conversions occur; the rate at which the convert (channel) clock is set.
Units: Conversions per Second (Channels per Second)

Interchannel delay
The amount of time which passes between sampling consecutive channels in the channel list. The convert rate is the reciprocal of the interchannel delay. The interchannel delay must be short enough to allow for sampling all the channels in the channel list within one sample clock period. The greater the interchannel delay, the more time your amplifier is given to settle before the next reading (see the settling time tutorial linked below).

For example code for data acquisition, please refer to the DAQ examples that install with your NI-DAQ installation or search for additional code from our Developer Zone.

Related Links:
KnowledgeBase 30LDURMV: Difference Between the Scan Clock (Sample Clock) and Channel Clock (Convert Clock)

KnowledgeBase 2XPE1QCW: How is the Convert (Channel) Clock Rate Determined in NI-DAQmx and Traditional NI-DAQ?
KnowledgeBase 1Z5D23PH: Documentation Resources for NI-DAQ 7.x
KnowledgeBase 1DAIG7W3: What Is the Difference Between Interval Scanning and Round Robin Scanning?
Developer Zone Tutorial: Is Your Data Inaccurate Because of Instrumentation Amplifier Settling Time?
Developer Zone


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Report Date: 09/07/2001
Last Updated: 08/31/2007
Document ID: 2D6CTML8

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