wrong rms lab view values

[ifyo]

hi, i am using labview to measure the rms value of the voltages, by sweeping the signal frequency from 10hz to 1MHZ,   lab view keep giving me the wrong rms value, but when i connect i manual digital multimeter to the circuit i can get the right rms voltages. can any one tell me hoow to solve this problem. find attached my vi.

[GerdW]

Hi ifyo,

you have set your DAQmx-Read to "N channels 1 Sample" - and you use that one sample to calculate a RMS value. What do you expect from such calculations?

As you only use one channel you could use "1 channel N samples" instead...

[ifyo]

hi, i had tried the option u talked about  and still i can not get the exact reading i am looking for.can some on e tell me what to do to resolve this

[ifyo]

can anyone tell me how i CAN RESOLVE THIS RMS VOLTAGE MEASUREMENT READING FROM LAB VIEW

[Dennis_Knutson]

Don't shout. Have some patience. Its a holiday. Describe the exact physical connections and the exact difference you see in your code.

[ifyo]

thanks dennis

      , i have the op amp 741 chip connected in non inverting mode to measure the gain bandwidth product. the function generator is connected to pin three(3) of the op amp 741 and the two resitor RF(feed back resistor=1Mohnms and R1 resitor of 100kilo ohm which when the two resistors are combined together, it will give a gain of 10. the ouput from the op amp which is pin 6 is connected to a10 to acquire the signal the rms voltage once i start increasing the frequency from 10hz to 1Mhz.

the lab view is giving me rms voltage value of 5.3v for all the frequency ranges while  if i connect a multimeter to the a10 , i will be getting rms voltages readings of 3.40, 3.65,3.64, 3.62, 3.60, 0.089volts  for 10hz to 1Mhz frequency range which will give me th actual reading.

[RavensFan]

I see two problems with your code.

1.  You've set the acquisition rate to 1000 Hz.  As you increase the frequence on your Elvis board, that is not going to be enough acquisition rate to adequately measure your output frequency.

2.  You are using 1 channel N samples, but you didn't wire up anything to samples.  So it defaults to -1.  The help for the DAQmx read says

"If the task acquires a finite number of samples and you set this input to -1, the VI waits for the task to acquire all requested samples, then reads those samples. If you set the Read All Available Data property to TRUE, the VI reads the samples currently available in the buffer and does not wait for the task to acquire all requested samples."

So if you are only readying 1000 samples, that is not enough for your higher frequency signals.

[ifyo]

thanks Ravens Fan for your  reply,

    i have added couple of things you said i should add for me to get the exact rms voltage vlaues,i can't still get the lab view to give me the exact reading once i start sweeping from 10hz to 1Mhz an d with the multimeter i can see the right readings, is there any other solution to this  

[Dennis_Knutson]

You do not specify the terminal connection (Differential, RSE, etc). Is that an option? What is the actual hardware used? You mention where you have AI0 connected. What about the gnd or other input of a dfferential measurement?

[ifyo]

thanks dennis for your reply,

   i am using NI Elvis work bench for my experiment on gain bandwidth product of 741 op amp. the 741 chip has 8 pins,  pin 7 is connected to the +15vols supply from the NI elvis, pin 4 is conneted to -15 volts, the function generator from the NiELVIS IS CONNECTED TO THE NON INVERTING PIN THAT is pin 3 (Vin) and the the feed back resistor Rf=1Mohms  and R1 =100kilo ohms resistors are connected to the inverting pin of the 741 which pin 2, the second leg of the  the 100kilo ohms is connected to the ground and the feed back resistor (Rf) is connected to the output pin of the 741 which is pin 6 (Vout).  the output signal from the pin 6 is connected to A10  which is the anologue input on the Ni elvis instrument which i am using to acquire the signal generated from the function generator . find attached my 741 configuration.

 i am using the vi i have written  to increase the value of the function generator frequency from 10hz to 1Mhz and acquire the signal  and measure the rms voltage of the signals coming through the A10 which should give me a decresasing value of rms voltage each time i increase the frequency.

 with the resistor values i have, the gain of the circuit is 10 and the bAndwidth should be 100000 and when i multple the gain by the bandwidth, it will give me 1MHZ. WHICH IS THE GAIN BANDWIDTH PRODUCT OF A 741 OP AMP.