Latency (1-2 seconds) in Real-Time DAQ using PXIe-4302

[LVUser94]

I am using LabVIEW 2020 RT to read continuously from a 32-Channel PXIe-4302 at 100 Hz Scan Rate. My Loop rate is set to 100ms, and I ask for 10 samples per read. When I monitor that actual loop time, it is indeed 100ms and I am getting 10 samples per read. However the voltage values are typically lagging the actual voltage input (from a potentiometer connected to the channel). It takes > 1 second for any signal level change to appear in the data.

The same code (except that I am using a cDAQ-9201 8-channel DAQ module instead of the 32-Ch PXIe-4302) on LabVIEW 2020 (Windows) runs fine and any signal level change appears almost instantaneously in the data.

I have also played with the DAQmx Read Properties (Relative To Recent Sample, and Offset set to -1) but see no improvement in the RT. The 1s delay remains. 

I did some more tests today, but this time I used the NI MAX Test Panels instead of my VI, using the same settings for Scan Rate (100Hz) & Samples to Read (10).

With the MAX Test Panels (in RT), there is still the same 1-2 s latency. So does this indicate a driver issue or PXI card issue? Or is there any special settings needed in the PXI system to minimize the latency?

The DAQmx version on the RT PXI System is NI-DAQmx 21.0.0 .

Has anyone seen this noticeable lag in RT and not in Windows? 

I am enclosing a stripped down Main VI and its SubVI for your examination.

Thanks for your time!

[santo_13 · Accepted solution]

A part of the reason for the delay is the filter after the Delta Sigma ADC in the instrument, this delay is specified in the datasheet as shown below,

You can expect delays in the order of ~582ms

[LVUser94]

Thank you Santo_13. I was unaware of the filter delay in the ADC. That could very well be the reason.

Can the filter be disabled (using DAQmx Properties in LabVIEW)?

[LVUser94]

By reading the manual closely it seems that the digital filter delay is specified in # of Samples (i.e., not a fixed time value like 50 ms for example)

So the delay can be reduced by just increasing the scan rate. If i increased the scan rate from 100 Hz to 1000 Hz, I can potentially reduce the delay by a factor of 10. Does this make sense?

[santo_13]

Yes, you can reduce the absolute time by increasing the sampling rate.

About the digital filter delay, it cannot be removed it is required for a Delta-Sigma ADC to function as designed.

[LVUser94]

I set the Scan Rate to 2000 Hz. That  minimized the latency to unnoticeable levels).

Thank you Santo_13!