Multifunction DAQ
40 cycle offset of data between NI9215 and NI9229 modules
Hi Holroyd
The 24-bit resolution Analog Input (AI) cDAQ modules use a delta-sigma
converter and an oversample clock to read the analog data. There is an
inherent input delay due to built-in analog and digital filtering. If
the module uses simultaneous sampling, each of the channels on the
24-bit module will be synchronized. However, if you are also reading
from an AI module that has a lower resolution there will be a finite
offset between the 24-bit resolution module and the lower resolution
module.
The delta-sigma input delay can be found in the User
Guide and Specifications for the 24-bit module(s). For example, for sampling rates (fs) at or below 25kS/s has an input delay of 12.8/fs. For faster sampling rates, the
delay is 9.8/fs. Refer to the specifications for the valid data rates
available for the internal sample clock.
To align the signals
for analysis or display, you can shift the signal(s) by replacing the
t0 of the waveform(s) by an amount equal to the offset.
In LabVIEW, you can replace t0 of a waveform using the Get Waveform Components and Build Waveform functions. The code in Figure 1 demonstrates how you would use these functions with the NI 9233:
Figure 1
You could perform a similar operation using the input delay formula for any other module. Here is a before and after example of how your signals may look:
Figure 2
As shown in Figure 2, the subtraction of filter delay from the t0 of the DSA waveform is most helpful when plotting DSA and DAQ waveforms simultaneously. If saving the waveforms to file is most important, then removing samples is the most effective solution. For example, assume the filter delay is 12.8/fs and X is the amount of samples that occured during the filter delay. To properly align the waveforms, the first X samples need to be removed from DSA Waveform while the last X samples need to be removed from the DAQ waveform.
Please let me know if this is of help and if this works. Thanks!
