Dmm Aquisition problem

[Danut81]

I'm using a 4070 Dmm from a PXI 1010.

I'm trying to make a waveform acquisition of a Voltage and I want to measure the rising front of the signal.

The period of time that I'm interested in measured with the digitizer is arround 500 us.

I configured the digital resolution to 3.5 and sample rate to 1800000.

I read the signal but my transition was done in only 4-5 samples wich is not god enough for me.

Iif someone can give a hint or something please d'ont hesitate.

[MicrowaveDave]

Hey,

      It looks like we need to set up triggering for the DMM.  If I understand correctly, by the time the acquisition starts, the signal has almost completed its transition.  Also, note that the acquisition time frame consists of several components.  These are:

1) Switch Time (if switching is used)

2) ADC calibration (if enabled)

3) Auto-Zero (if enabled)

4) Settling time

In order to make the first measurement more quickly, make sure that you have not enabled ACD calibration and AutoZero.  Otherwise, a little more information about how the acquisition is currently being triggered would be helpful.

Regards,

David Hall | Applications Engineering | National Instruments

[MicrowaveDave]

Actually, to add a little more information.  When sampling at 1.8 MHz, you'll need to collect about 900 samples to get 500 us of data. 

Thus, make sure you configure the acquisition to acquire no more than 900 samples, which you can do with the 'niDMM Configure Waveform acquisitionsfunction.  By limiting the waveform aquisition acquisition only 900 samples, we prevent the driver from allocating a much larger buffer, which takes longer.   This is important for your measurement becuase because event is happening within 4-5 samples of the first data point. 

Also, remember that the NI-4070 is capable of digital triggering.  Thus, if the event you are capturing happens syncrhonously synchronously a digital TTL pulse, you can use that pulse to start your aquisition acquisition well.  If you're just getting started, a good example you might want to check out can be found at: "C:\Program Files\National Instruments\LabVIEW 7.1\examples\instr\niDMM\Waveform Acquisition".  Hope this helps!

Regards,

David Hall | Applications Engineering | National Instruments