The NI 449x and 446x DSA modules are high-sensitivity cards that are meant to operate in specific conditions. If you are measuring low-noise acoustic signals, the fan noise of a chassis with a fan underneath the card slots can interfere with measurements.
Chassis such as the PXIe-107x series, the PXIe-1083, and the PXIe-1088 have fans on the bottom which introduces electrical and mechanical noise that the modules pick up. This raises the noise floor of the DSA card causing it to be out of specification inside these chassis.
It is recommended to use a high-performance chassis (like the PXIe-1092
) or any chassis where the fans aren't located on the bottom of the chassis.
If you want to lower the noise further, make sure to place the DSA cards as far from the controller slot as possible.