Digital Multimeters (DMMs) and Precision DC Sources

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DMM (NI 4070), how to properly set AC Freq (bandwidth) per sampleRate

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using an NI4070 DMM, and I see the max bandwitdh is 300kHz per the specificiations.  But I don't fully understand how to set the acFrequency min and max, according to the sample rate or reading rate.

 

at 6 1/2 digits of resolution, the reading rate can vary from 0.25 S/s to 100 S/s, and this range corresponds to a lower end on the bandwitdh (acFreq minimum) from 1 Hz to 400 Hz.

 

(Q1a) - is the reading rate controlled by the IviDmm_ConfigureACBandwidth minimum setting?   or vice versa?  

 

otherwise, i dont see how to control either the reading rate or the sample rate.   IviDmm_ConfigureMeasurement only allows you to control the range and resolution.  

 

(Q1b) - is there a way to directly control either the Sample Rate (digitizer) or the Reading Rate (dmm)?

 

 

(Q2) - the upper limit of the AC bandwidth always seems to be at 300kHz...  is there ever a reason to reduce that maximum value?



(Q3) - Finally, unlike the traditional niDmm function, the resolution via the IVI configuration needs to be passed as an absolute value; does it correpsond directly to the number of digits and the range?   For example if I want 6 1/2 digits at 300V range, i assume by the specs that the resolution should be set 0.001 V...   followign this, if i want 5 1/2 digits at 1V range, should the resolution be set to 0.00001 V?

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Accepted by topic author rjohnson

Hi Rjohnson,

       I'll try to answer your questions as well as I can:

Q1a.  The ConfigurACBandwidth function is used by the NI-DMM driver to calculate the proper aperautre for the measurement.  So yes, by adjusting your minimum frequency, you will affect your reading rate. 

 

Q1b.  Your reading rate is going to depend largely on your measurement cycle.  To get a faster measurement cycle, there are a few things that you can adjust.  You can programmatically control your aperature time, as well as your settle time.

 

Q2. I cannot find a reason to change it. This parameter is used only for error checking and verifies that the value of this parameter is less than the maximum frequency of the device.

 

Q2b.  I believe that what you said was right, but I'll need to check on that- I'll let you know soon.

 

Hope that helps.  I would recommend checking out the explanation of the DMM Measurement Cycle in the DMM Help» Devices»NI 4070»DMM Measuments»DMM Measurement Cycle.

Take care!!

 

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thanks for the reply.  

 

i'll check out that link and look for any clarification you might add.

 

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Hi rjohnson-

        I checked on your Q3, and you are correct. You'll want to calculate the resolution based on the specs like you had done (according to the table in the specs, with each digit less of resolution corresponding to a drop in resolution by a factor of 10, so 5 1/2 digits and 1V range= .00001).  Let me know if there are other specific questions you have.  Have a great day!

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