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.