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.