NI PXI - 4071 settings

[E351635]

Do anyone know the dependecies between the reading rate, apperture and settling time to get best possible measurements. E.g. when I want to measure DC voltage on 10.0 V range with sampling rate 8000 Samples / s what should I set first (apperture or settling time) and to which values ??? Is this sequence correct ???

if( Rate < 7 )                                      
   niDMM_ConfigureMeasurementDigits(Handle, NIDMM_VAL_DC_VOLTS, 10.0, 7.5);
else if( Rate < 100 )
  niDMM_ConfigureMeasurementDigits(Handle, NIDMM_VAL_DC_VOLTS, 10.0, 6.5);
else if( Rate < 3000 )                                                                         
  niDMM_ConfigureMeasurementDigits(Handle, NIDMM_VAL_DC_VOLTS, 10.0, 5.5);
else
  niDMM_ConfigureMeasurementDigits(Handle, NIDMM_VAL_DC_VOLTS, 10.0, 4.5);
        
                                                                                        //NIDMM_VAL_IMMEDIATE, 0);
niDMM_ConfigureMultiPoint (Handle, triggers, samples, NIDMM_VAL_INTERVAL, sample_time);

now set apperture and settling time ??? initiate and fetch_multipoint.         

[Brian F.]

Hello. 

Thank you for contacting National Instruments. 

I read through some of the LabVIEW help files on these functions and you are definitely correct in your order.  The Configure Measurement  operation must be done before the Aperture Time.  This is stated in the help files for the LabVIEW help file.  Then, I would configure the Settle Time, Initiate, and Fetch.  In terms of specific values for aperture time and settle time, this can be application specific.  However, there are a few passages below that contain good pointers on setting these parameters:

"Aperture time is the period during which the ADC is reading the input signal. By increasing the aperture time, we can potentially improve the accuracy of our measurement because we are measuring for a longer time and internally averaging the samples together. The aperture time can also be set to multiples of the power line frequency so that it rejects those unwanted frequencies. Typical power line frequencies are 60 Hz (16.67 ms) for the USA and 50 Hz (20 ms) for most other countries.  So for a 60 Hz system, an aperture time of 1 powerline cycle (PLC) or 16.67 ms is the minimum to provide line rejection. The aperture time can also be set with a property node."

Helpful Aperture Link
http://digital.ni.com/public.nsf/websearch/D99D0C3EE1E6846086257206003C0E6F?OpenDocument

Finally, a great resource for information about settling time can be found in the NI DMM Help, which can be downloaded here:

NI Digital Multimeters Help
http://digital.ni.com/manuals.nsf/websearch/3CBF687DF86E450B8625724B006BBE5F

Simply search for settling time and there will be a lot of information on what the recommended settling times are. 

Let us know if this answers all your questions.  Have a great day!

Brian F
Applications Engineer
National Instruments
 

[E351635]

Thank you, this is nice however:

I would like to create a function that will have just one argument - the rate of my measurement. It will run an infinite measurement (number of triggers set to zero). And I want the function (it will always set the function: DC VOLTAGE measurement, range: 10V) to take the sample time (it is calculated from the argument: sample_time = 1 / rate) as priority. I know that measurement_time = sample_time = switch_time + settle_time + apperture_time + ADC calibration + AUTO Zero + ..... . OK let's say that I will turn off everything not really neccessery (ADC calibration, AUTO zero, also in MAX(Range check, Record Coercions, ....)), that means that my sample_time will depend only on switch time, settle time and apperture time. I have found out that (the values are in seconds) when I set the apperture time from 0.000009 to 0.000020  the switch time will be 0.000076; from 0.000021 to 0.00500  the switch time will be 0.000121 and from 0.00501 ++ the switch time is 0.000284.

So what I need to know now is> what if I set the settle time to zero (than I can calculate the apperture, switch time and set them to proper values), but wouldn't the measurement reasults be much better if I will set the settle time to the biggest possible value and leave the apperture + switch time much smaller ??  Or is the middle way the better choise.

Therefore if somebody could tell me what is the best way (should I set the settle time = 0.2 * sample time      and then calculate the rest  or should I should I try to set values so they will best possibly match the apperture time = 2*settle time ) ??? So once again what are the dependencies between apperture and settle time for this instrument in most cases (most robust solution)???     

[Brian F.]

From your description, it sounds like you want to operate the 4071 in digitizer mode.  In digitizer mode, you will actually be able to achieve up to 1.8 MS/s acquisition rate because the averaging, filtering, and weighting functionality is removed.  Using the Configure Waveform Acquisition VI, or its text based equivalent, you can specify an acquisition rate and most of the other parameters will no longer be an issue.   

Brian F
Applications Engineer
National Instruments

[E351635]

Yes, I would love to use the digitizer mode, but as I said I need an infinite measurement and digitizer mode supports measurement that longs only 149 seconds.

[Brian F.]

In terms of your specific application, what rate or rates are you looking to sample at?  Can you talk about your application a bit so we can gain a better understanding of what you are trying to accomplish?  This will then shed light on why you want to sample for an infinite amount of time as well as facts we need to take into account while taking the measurements. 

Brian F
Applicaitons Engineer
National Instruments