Synchronizing NI c Series Delta Sigma Modules: Latency between NI 9252 and NI 9242

[ChrGra]

Dear Community,

I've been working on a data acquisition project with a cRIO 9045 that requires me to measure phase voltages (NI 9242) and phase currents via a hall effect sensor acquired by the NI 9252 module. My sampling rate is 25kHz with a sample number of 500.The issue I'm having is that there is always a ~1ms latency between the voltage and current signals.I've already attempted to set up the NI signal based synchronization tasks (solutions in figures 5 and 8), but I'm still getting the error. The issue is definetly a software problem, since I am measuring the correct signal at the analg inputs of both

I'd appreciate it if someone could give me some advice.

Please find my project file attached.

Thank you for your support!

Christopher

[rolfk]

No, the problem is a hardware problem. Delta Sigma converters have an inherent delay since the converter takes a certain number of samples, depending on the accuracy, from when it appears at the analog input until the according digital value appears at the output. I'm not sure if DAQmx has means to automagically compensate for that. In the past I used PXI hardware with that kind of ADCs and compensated for it on the application side by calculating the actual delay of samples and shift them accordingly.

[ChrGra]

Hello Rolf Kalbermatter,

thank you for your reply.

I am aware, that the group delay of the modules is causing the problem.

Due to my previous research I've learned, that this delay can be removed programmatically by DAQmx timing VIs?

Best, Christopher

[mcduff]

Since both modules are in the same chassis, and have the same sampling rate, why not use Channel Expansion. Combine channels from both devices into a single task.

I don't have a cRIO here, but I got no errors from simulated devices and a cDAQ chassis. You will still have to account for the group delay as a cDAQ does not have the capability to remove it as some PXI boards do.

[ChrGra · Accepted solution]

Hi, thank you for your reply.

The channel expansion is unfortunately not possible, since the modules can't be syncrhonized in NI MAX, which leads to the before mentioned group delay between both modules.

Anyway, I've found a propper solution: Reading the AI.FilterDelay from both DAQmx Tasks and synchronizing them by the DAQmx Tining vi works pretty fine.

[mcduff]

I can create a task in MAX that uses both devices. Not sure what the problem is. You don't need to create a task in MAX, just use DAQmx functions in LabVIEW.