How Can I Compensate for Different Group Delays with C Series Modules in LabVIEW FPGA?

Updated Nov 15, 2017

Issue Details

I am acquiring data from a NI 9215 and NI 9229 in LabVIEW FPGA. When synchronizing both of these modules and feeding in the same signal to both of them, I see a phase difference between them. I know that this is due to the delta-sigma ADC used on the NI 9229, but how can I compensate for this?

Solution

Group delay is a trait inherent to delta-sigma analog-to-digital converters (ADC).  All of the NI C Series modules which use a delta-sigma ADC have a specification for group delay in the User Manual.  Group delays are typically different for each ADC and can vary depending on the 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. 

Additional Information

Filters are used to add an artificial delay so that the total delays on both modules are equal.  For example, the NI 9215 has a group delay of 0 samples because it uses a SAR ADC.  On the other hand, the NI 9229 has a group delay of 40.0 samples due to its delta-sigma ADC.  The number of samples in the group delay can be seen in the specification Input delay, in the specifications documents in the Related Links section below.  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.