What "Maximum AI Sample Rate Single Channel and Aggregate" Values Means?

Updated Oct 11, 2018

Reported In

Hardware

  • Multifunction I/O Device

Issue Details

When I look at the user guides or specification documents of some NI data acquisition devices (DAQs), I see that 2 different sampling rates are available: single channel and multiple channels (aggregate). What is the meaning of these two types of channels and how does it affect the sampling rate or sampling speed?

Solution

Here an example from the USB-6008/6009 User Guide :
 

Some analog acquisition devices (such as the USB-6008, USB-6009, USB-6001, USB-6002) only have 1 ADC (Analog to Digital Converter). When you read from more than one analog input channel, the maximum sampling rate of the ADC must be shared between all active channels, as this image shows :
 
 
So, for an ADC with a sampling frequency Fs, if N channels are active, the maximum sampling frequency attainable for each channel is Fs/N.

For example, for the USB-6009 (reference above table):
  • Maximum sampling rate of one channel, Fs_1channel = 48kHz
  • Maximum sampling rate of two channels, Fs_2channel = Fs/2 = 24kHz
  • Maximum sampling rate of N channels, Fs_Nchannel = Fs/N = 48/N kHz

Additional Information

Aggregate means that the sampling rate that is specified will be shared through all the channels. In other words, the sampling rate shows the amount of samples that can be sent from the ADC to the bus, which must be shared depending on the amount of active channels. Read more about it here: Analog Acquisition Slows Down With Each Additional Channel

The easiest way to know if a National Instruments sampling rate is aggregate is if the specification is written as S/s (samples per second), if you read S/s/ch (samples per second per channel), it means that the speed specified is individual for each channel - usually meaning the card has one ADC per channel. Learn more about this difference here: Multiplexed Versus Simultaneous Sampling

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