Digital Multimeters (DMMs) and Precision DC Sources

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pxi 4065 fast single point AC measurement

I have the following setup using "niDMM Initialize.vi", "niDMM Configure Measurement Digits.vi" and "niDMM Configure Powerline Frequency.vi" to initialize the DMM.

Each measurement, acquired with "DMM read.vi" is taken and used in a while loop.

My problem is that each AC measurement takes about 1000 ms, whereas a DC measurement only takes about 30ms.

How do I get a rate of 50-100 measurements/s as mentioned in the datasheet?

Is that possible at all without using multipoint measuring?

 

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Hi ddrayss,

 

First of all, would you mind uploading your images again?  They are not showing up in your post.

 

As the specifications document states, you can only expect 50-100 S/s if you are operating at 5 1/2 or 4 1/2 digits of resolution.  How many digits of resolution are you using?

 

There are a few things I would suggest to speed up your acquisition.  First of all, when you are trying to maximize your reading rate, you should use the VI's from the "Fetch" subpalette instead of "niDMM Read".  The second bullet point in this article provides a brief overview of the difference between the two functions.  This page from the NI-DMM Help goes into further detail.  

 

Basically, for a continuous measurement you need to add a few VI's to your program to increase your measurement speed.  Before the While Loop, add a niDMM Configure Multipoint VI.  Set the Sample Count input to 0 if you are going to be measuring continuously, otherwise set it to the number of samples you want to read.  Before the While Loop call niDMM Initiate to start your acquisition.  Inside the While Loop you can call niDMM Fetch or niDMM Fetch Multipoint to return your data.  Fetch only returns one value at a time while Fetch Multipoint will return an array of values.  After your While Loop has finished executing call niDMM Abort to stop the acquisition.  

 

Also, if you look at the default aperture time for the 4065, you can see that the aperture time is a function of the Minimum Frequency property when configuring your measurement for 5 1/2 and 4 1/2 digits of resolution.  The Minimum Frequency property has a default of 20 Hz, resulting in a 500 ms default Aperture Time at 5 1/2 digits and 250 ms default Aperture Time at 4 1/2 digits.  If the minimum frequency of your input signal is greater than 20 Hz, you can achieve a faster reading rate if you increase the minimum frequency setting with an niDMM Property Node.  The property is located under Configuration > Measurement Options > Min Frequency.  You will need to change this property before you call your Initiate function.

 

Let me know if this helps!

 

 

Regards,

Jared R.
Precision DC Product Support Engineer
National Instruments
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Hi JaredRo,

 

thank you very much for your detailed answer.

The two pictures are posted again.

 

Actually I'm using 4 1/2 or 3 1/2 digit.

 

I think, I'll try out the solution with triggered multipoint measurement and the fetch vi for single data acquirements.

As you see I'm measuring 0-300 AC volts at 50hz.

 

"If the minimum frequency of your input signal is greater than 20 hz, you can achieve a faster reading rate if you increase the minimum frequency setting with an niDMM Property Node.  The property is located under Configuration > Measurement Options > Min Frequency."

 

Did you meant this vi: "niDMM Configure AC Bandwidth.vi" ? - I couldn't find any other similar.

 

 

ddrayss

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