Analog Acquisition Slows Down With Each Additional Channel

Updated Aug 2, 2018

Reported In

Other

Multifunction DAQ(MIO), Single-Board RIO(sbRIO)

Issue Details

Why does the read speed of each AI (analog input) channel decrease as I increase the number of AI channels I am reading from?  How do I read from multiple AI channels at the same time, simultaneously?  I have seen this behavior on sbRIO (single board RIO) as well as DAQ cards.

Solution

If you find that it takes longer to read from each AI channel with adding more AI channels, it indicates that your device is most likely using analog input channels that are multiplexed. This means that your device only has a single ADC(analog digital converter), so each AI channel has to be processed individually and each additional channel will decrease the channel sampling rate, as shown in Figure 1 and Figure 2. This is assuming that you are reading the channels correctly in software and not introducing significantly overhead from the software architecture.
 

Figure 1. Simultaneous Sampling


Figure 2. Multiplexed Sampling
 
For example, in the following diagram of an X-series Multifunction DAQ device's internal circuitry, you can see that there are multiple channels that are all multiplexed(MUXed) through a single ADC. The increase in channels increases the amount of time that the device multiplexes, decreasing the sampling rate, as shown in Figure 3. For a more in depth look at the X-series circuitry, see the X-Series Manual.
 

Figure 3. X-Series Device Internal Circuitry 

If you need to read from multiple AI channels without a decrease in sampling rate with additional channels, then you need to get a device that has simultaneous AI sampling.
 

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