How to set analog input (anti-alias) filter on 6281?

[JeffDM]

Manuals say there is programmable analog input filter that can be turned on/off (40kHz/750kHz). I cannot figure out how I set the filter?

[Zaboulou]

Hi,

To enable filter on NI-6281, you have to create a channel (DAQmx -Create channel.vi). The property node "AI.LowPass;CuffFreq" allows you to set the filter. First af all, you have to enable filter. See below example (LV 7.1).

Regards,
Zaboulou

[JeffDM]

Thanks for the help. This works for me with 1 input channel but when I try to enable the filter on a task with 2 seperate input channels, the 2 inputs show the same voltage. They are different with the filter disabled (one traces physical position while the other is constant as expected). Is it valid to enable the filter for an entire task?

[Tom_W_[DE]]

Hi Jeff-

Enabling the filter will affect all channels operating on the board, so it is valid to select the filter for an entire task.

The syntax for adding multiple channels to an analog input task is shown in the attached VI. Please let us know if this clarifies the operation.

Thanks and have a great day!

Message Edited by Tom W. on 06-06-2005 05:26 PM

[JeffDM]

This appears to be happening when reading multiple channels because the settling time of the filter is greater than the period between two conversions on the analog-to-digital converter (ADC). The workaround is to use external lowpass filters.