Issue Details

At a higher level, the specifications of the NI 9233 and NI 9234 seem to be quite similar. What are some of the key differences between both modules?

Solution

The following points outline the main differences between the NI 9233 and NI 9234:

Phase Linearity
The 9234 has better phase linearity than the 9233. The phase non-linearity of the 9233 means signal components at different frequencies will appear slightly out of phase (by as much as a couple of degrees) relative to each other. Although this is not significant for most machine monitoring applications, it might be a problem for testing professional audio equipment.

IEPE 
IEPE is always enabled on the 9233, but the 9234 has it selectively enabled per channel. Having IEPE always enabled means that the 9233 is meant for connecting to IEPE compatible sensors. Since excitation is always enabled on the 9233, it might not be suitable for non-IEPE sensors. In particular, any sensor that connects to the 9233 must accept the 2mA constant current excitation sourced from the 9233.  The 9234 has the ability to turn off excitation on a per channel basis, which makes it compatible with sensors that are not able to accept this current.

It is not necessary to use IEPE sensors on either of these devices. These devices are specifically designed for use with IEPE sensors, but will also work with a non-IEPE BNC connector. 

AC Coupling 
Similarly to the IEPE,  AC coupling is always enabled on the 9233, but the 9234 has programmable, per channel,  AC/DC coupling. If you are planning to do DC measurements, use the NI 9234 in DC coupled mode.  The 9233 has a high-pass cutoff frequency (-3dB) of .5 Hz, so any low-frequency signals in your signal will be attenuated and any DC shift in your AC signals will also be removed.

Phase Delay
In order to synchronize the 9233 or 9234 with other DSA modules you need to compensate acquired data by referencing the phase delay specs. The different phase delay specs for the 9233 and 9234 are below:

9233 ..................... 12.8 ÷ fs (for ≤ 25 kS/sec)
9233 ..................... 9.8 ÷ fs (for >25 kS/sec) 
9234 ......................38.4 ÷ fs + 3.2 μs 


Native Sampling Rate 
Valid Sampling Rate = (Master Timebase) / (256*n), where n is an integer from 1 to 31

The NI 9233 has a 12.8 MHz Timebase, giving a maximum sampling frequency (n = 1) of 50.0 kS/s. The NI 9234 has a 13.1072 MHz Timebase, giving a maximum sampling frequency ( n = 1) of 51.2 kS/s. The 51.2 kS/s of the 9234 may make it more desirable for some audio applications. 

The 9234 can either use its native sampling rates or it can synchronize to one of the other modules. So if you synchronize a 9237 and a 9234 together, you can either set the 9234 as the master and get 51.2 kS/s for both modules (or 25.6 kS/s or other divisors), OR you can set the 9237 as the master and get 50.0 kS/s for both modules (or 25 kS/s, etc.).

Noise Attenuation
The 9234 has slightly better noise attenuation, especially with crosstalk between paired channels (1 and 2, 3 and 4).