Delta-Sigma C Series, DSA and SC Express devices have discrete sample rates because of Delta-Sigma Analog to Digital Converter (ADC)
. These require oversampling, which means they require their own Sample Clock Timebase. These timebases can only be divided down by discrete numbers.
Taking again the NI 9234 and NI 9237 as an example, while both devices use the same formula, because the internal timebases are different (13.1072 MHz for the NI 9234 and 12.8 MHz for the NI 9237), the valid sample rates will differ. If you set a sample rate of 2.4 kS/s on each device, the NI 9234 sample rate will be coerced to 2.438 kS/s and the NI 9237's to 2.5 kS/s (both with n = 21
) because those are the next values that result from the formula.
If you use an NI-9237 in the same task as an NI-9234, both will run at the 9234's rates by default. This is because DAQmx synchronizes multiple modules by automatically routing the faster master timebase to the other modules. However, you can force the task to use any of the available timebases by manually setting the Sample Clock Timebase Source property using the DAQmx Timing Property Node: