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How to Compensate for Different Group Delays with C Series Modules in LabVIEW FPGA

Primary Software: LabVIEW Modules>>LabVIEW FPGA Module
Primary Software Version: 8.2
Primary Software Fixed Version: N/A
Secondary Software: N/A
Hardware: CompactDAQ>>Analog Input Modules>>NI 9215, CompactRIO>>Analog Input Modules>>NI 9215 BNC, CompactDAQ>>Analog Input Modules>>NI 9233, CompactRIO>>Analog Input Modules>>NI 9239, CompactRIO>>Analog Input Modules>>NI 9237

Problem:
I'm acquiring data from a NI 9215 and NI 9229 in LabVIEW FPGA.  Synchronization between channels is important to my application. When I feed the same signal into both modules, I see a phase difference between them.  I know that this is due to the delta sigma ADC used on the NI 9229, but am not sure how to compensate for it.

Solution:
Group delay is a trait inherent to delta sigma converters.  All of the NI C Series which use a delta sigma converter have a specification for group delay in the User Manual.  Group delays are typically different for each ADC and can sometimes be different at different sampling rates.  A method to compensate for these differences in group delays is to use a filter which delays a signal by a fractional number of samples. 

Filters are used to add delay to the module with less delay so that the delays on both modules are the same.  For example, the NI 9215 has a group delay of 0 samples(not a delta sigma ADC) while the NI 9229 has a group delay of 40.0 samples.  By creating a filter with a delay length of 40.0 samples and applying it to the NI 9215 data,  both modules will have the same delay and the data will be synchronized.

Attached below are some fractional delay filters designed using the Digital Filter Design Toolkit in LabVIEW which have delay lengths used for some common module combinations.  Please read the readme file associated with each filter.

Related Links:
KnowledgeBase 2UI8PGX4: Why Is My Data Delayed When Using DSA Devices?

Attachments:

Group Delay Filters for LabVIEW FPGALV FPGA Group Delay Filters.zip

Report Date: 09/11/2007
Last Updated: 06/16/2011
Document ID: 4DAEUNNQ

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