wrong rms lab view values

[Henrik_Volkers]

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@ifyo wrote:

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.

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Two points, because it's for edjucation 😉 :

Recalculate the gain you have setup.  Shure it's 10??

Take a look at the multimeter specifications, will it measure 1MHz AC (Some do, some just go up to 100kHz...) ?

The point of setting the correct samplerate (Shannon, Nyquist) is already mentioned.

@UUps your DAQ don't sample @ 2MHz? How about building your own rectifier and TP and measure DC and learn about crest factor?? Or have a look how the Multimeter does it 😉

It's a nice project, and the 'good' old 741 is a perfect candidate (if you have a original 741!  Because they are hard to find these days, mostly more modern designs just labeled 741)

[ifyo]

THANKS FOR YOUR REPLY Henrik Volkers.

     am trying to measure the rms voltage value of the signal, sinec the experiment is for gain bandwidth product, the more i increase the frequency on the function generator the more the signal on the output pin 6 becomes smaller, so at 1MHZ the output signal will be far small than the input signal from the the function genertor and i can now measure the rms voltage value of the signal. which i am getting a wrong reading.

[johnsold]

You certainly do not need an rms measurement to calculate the gain-bandwidth product.  You just need to use a consistent and accurate measurement technique on both the input and output so that you can calculate the gain at various frequencies.  If you look at the output of the amplifier with an oscilloscope, you will not have an rms measurement (although many scopes can calculate rms).

Do you see clean sine waves on the Waveform Graph? Have you measured and plotted the output of the function generator over the frequency range?

As Henrik mentioned, many voltmeters cannot accurately measure frequencies above a few tens or hundreds of kHz.  Is your meter accurate over the frequency range?

Does your function generator provide a DC path for the bias currents at the input to the 741?  Typical bias current is 80 nA while maximum bias current can be as much as 0.5 uA.  Without bias current the device will not operate correctly.

Lynn