Interval Scanning, Simultaneous Sampling, and Round-Robin Scanning Support Hardware: Multifunction DAQ (MIO), Legacy DAQ Devices, SCXI, SCC
Problem: I am developing an application using some of your data acquisition boards on which I am acquiring from multiple channels. It is very important for me to know exactly how the different channels are getting sampled. Are they scanned one after another or are they sampled at exactly the same time? Which board supports which scanning type? Solution: There are essentially three types of scanning supported by National Instruments acquisition devices: Interval Scanning (most E-Series devices): 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 (S Series, DSA boards, 5101, 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. Round-Robin Scanning (PC-LPM-16, PC-LPM-16PnP, PC-516, DAQcard-500/516, Lab-LC, NI 4060): 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 channel is constant. Related Links: KnowledgeBase 2D6CTML8: Data Acquisition Sampling Terminology Attachments:
Report Date: 10/06/2004 Last Updated: 08/19/2005 Document ID: 3E59TDPK |
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