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Sampling frequency with DAQmx in .NET

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I am making my first application with DAQmx and .NET and have an issue with setting sampling frequency.

 

The setup is

Rack: 9188

Slot1: 9237

Slot2: 9237

Slot3: 3201

Slot4: 9421

 

My idea was that all channels are set up as global channels in NI MAX. In this way a channel can be configured with the right custom scaling and test run in MAX.

 

The .NET program then creates a Task, and adds the right channels to it (the _channels object is just a container for all objects):

Dim analogTask As New DAQmx.Task
_channels.analogTask = analogTask
_channels.LoadChannel = analogTask.AddGlobalChannel(config.loadChannelName)
_channels.TorqueChannel = analogTask.AddGlobalChannel(config.torqueChannelName)
_channels.TransverseForceChannel = analogTask.AddGlobalChannel(config.transversalForceChannelName)
_channels.TransversePosChannel = analogTask.AddGlobalChannel(config.transversalPosChannelName)

LoadChannel, TorqueChannel and TransverseForceChannel are Bridge type channels with custom scaling. The TransversePosChannel is Voltage type, also with custom scaling.

 

The sample clock is then configured and task is verified:

analogTask.Timing.ConfigureSampleClock(String.Empty, config.frequencyHz, _
    DAQmx.SampleClockActiveEdge.Rising, DAQmx.SampleQuantityMode.ContinuousSamples, config.frequencyHz)

analogTask.Control(DAQmx.TaskAction.Verify)

I then create an AnalogMultiChannelReader and a callback. NUM_SAMPLES_PER_READ is 500.

_analogReader = New DAQmx.AnalogMultiChannelReader(_analogTask.Stream)
_analogCallback = New AsyncCallback(AddressOf AnalogInCallback)

_analogReader.SynchronizeCallbacks = True _runAnalogSampling = True _analogReader.BeginReadWaveform(NUM_SAMPLES_PER_READ, _analogCallback, _analogTask)

 The callback looks something like this:

 

Private Sub AnalogInCallback(ByVal ar As IAsyncResult)
	Try
		Dim timeLoadData() As measuredData

		If ar.AsyncState Is _analogTask Then
			_daqData = _analogReader.EndReadWaveform(ar)
                         ... ... ...
... ... ... beginAnalogRead() End If Catch ex As Exception Try stopAllTasks() Finally RaiseEvent ErrorOccurred(Me, "DAQChannels.AnalogInCallback: " & ex.Message) End Try End Try End Sub

The issue is that the sampling frequency seems to be about 1.6 kHz no matter what I set in the ConfigureSampleClock call.

I have verified that 500 samples are returned at each call to the AnalogInCallback, but it is always called about three times / sec no matter what freqyency I have set.

 

I might have missed something obvious, but right now I am kind of stuck so I'd really appreciate your help here.

 

Linus Nilsson
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Solution
Accepted by topic author LinusN

Hello LinusM,

 

 

I do not see anything wrong with your code.

 

But please find the supported sampling rates for 9237 here:

http://digital.ni.com/public.nsf/allkb/593CC07F76B1405A862570DE005F6836

 

As you can see, minimum sampling rate is 1.6kS/s.

 

Can you please verify whether or not you are seeing the issue for higher sampling rates as well? For example @ 25 kS/s?

 

Thanks

 

Regards,

Christopher

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Hi

 

Just as I expected the answer was incredibly simple! I kind of knew I missed something vital there.

Well, the solution is simple; I'll just run at 2kHz and discard 3 out of 4 samples.

 

Thank you!

Linus Nilsson
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