Dynamic Signal Acquisition

cancel
Showing results for 
Search instead for 
Did you mean: 

PXI-4461 Settling time after gain change

Solved!
Go to solution

Hello all,

I've noticed that when switching between different gains (min/max input limits) on the PXI-4461 that my first few values aren't valid. Is there some sort of settling time that should be used when the hardware configuration is changed? I've been looking around through different manuals and I haven't come across anything to tell me what should be the appropriate amount of time to wait.

I'm attaching some of the results I'm seeing when using the NI MAX Test Panels when I go from a max input of 10V to 20V. I know that the quick solution would be to just toss the first samples, but my sampling rate isn't fixed so I won't always know how many samples to toss.

0 Kudos
Message 1 of 5
(5,378 Views)

Hello LGallegos,

 

I'm not sure if this is related to your issue, but you should use pseudo-differential for input configuration when you are using a DSA device. Let me know if that changes anything.

 

Cheers,

Saki K.
Applications Engineer
0 Kudos
Message 2 of 5
(5,235 Views)

problem is still observable with terminal configuration = pseudodifferential

 

btw: I can reproduce with a PCI 4461.

Doug
NI Sound and Vibration
0 Kudos
Message 3 of 5
(5,233 Views)
Solution
Accepted by topic author LGallegos_

Hi LGallegos_,

 

The behavior you are witnessing is a result of the device filter delay and the device using a free-running clock to drive ADC conversions. When a user configured acquisition is started there is a 'buffer' of samples that were acquired with the previous gain and filter setting prior to the configured acquisition start. However, software is applying the same gain scaling to all samples returned during the configured acquisition.

 

New DSA devices offered by NI have a built-in Filter Delay Removal feature that removes these samples from the returned acquisition. The PXI/PCI-4461 does not have this feature, but there are other methods of removing these samples from an acquisition or from display.

 

Method 1:

You can use the DAQmx Control Task VI prior to the DAQmx Start VI. By setting the DAQmx Control Task VI to commit you set the configuration of the DAQmx Task or acquisition on the hardware, so any potential switching for filter or gain settings should occur at this time. You can add a small delay between the DAQmx Control Task VI and DAQmx Start VI such that the 'buffered' data being acquired prior to the DAQmx Start VI uses the desired settings of the task.

Method 2:

As you mentioned you can remove/delete the samples from the beginning of the acquisition. You can address the concern of having a variable filter delay due to different sample rate settings by using the AI.FilterDelay property of the DAQmx Channel Property Node to get the expected number of filter delay samples.

 

Here is a good resource that covers the filter delay topic as well as partially demonstrates 'Method 2' described above:
KnowledgeBase - Why Is My Data Delayed When Using DSA Devices?

 

Best,

Jacob B.

Signal Conditioning PSE

Message 4 of 5
(5,223 Views)

Hey Jacob,

 

Thank you for the explanation and the resource! It was exactly what I was looking for.

 

Regards,

Luis

0 Kudos
Message 5 of 5
(5,213 Views)