Types of Scanning Supported by National Instruments Acquisition Devices

Updated Apr 11, 2018

Reported In

Driver

  • NI-DAQmx

Issue Details

I am developing an application involving multi-channel inputs and I need to know exactly how each of the channels are sampled. Are they sampled one by one or all at the same time? Also, how can I change the inter-channel delay when not sampling simultaneously? What does simultaneous sampling mean?

Solution

There are three types of scanning supported by National Instruments acquisition devices:
 

Interval Scanning

(most E Series devices, M series devices, some X series)
In this method, the sample clock is used to determine when to start acquiring samples and the convert clock is used to determine when each channel in the list is to be sampled. The time in between the sampling of two channels is called interchannel delay and can be set in software.

 

Simultaneous Sampling 

(all S Series, some X Series, DSA, PCI 6123, PCI 6115, PCI 6120, 5112, SCXI-1140, SC-2040)

With this method, all the channels are sampled exactly at the same time using sample-and-hold circuitry and an independent amplifier for each channel. The values are then multiplexed and sent to the ADC for conversion. S Series boards have an independent ADC for each channel.

Using the product selection filters on the multifunction DAQ product pages, you can narrow results to show all currently available simultaneous sampling devices. See the Related Links section below for this page. 

 

Round-Robin Scanning 

(PC-LPM-16, PC-LPM-16PnP, PC-516, DAQcard-500/516, Lab-LC, NI 4060, M series devices):
In this method, an analog multiplexer is used to alternatively connect each channel to the amplifier and the ADC. There is no sample clock. Only the convert clock is used to switch between one channel and the next one in the list, or from the last one on the list to the first. The delay between the channels is constant.

Additional Information

Refer to the NI DAQmx help document for more information about Multiplexed Versus Simultaneous Sampling.

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