how to determine the slew rate of an amplifier using lab view

[ifyo]

gud evening all

I have an op amp circuit built on the NI ELVIS board and I input a square wave from function generator to the circuit. I have to determine the slew rate of the op amp.In the labs, one would run the square wave over a range of frequencies and stop when the square wave looks like a triangular wave. At that frequency, one would then measure the slope of the signal and that will give you the slew rate. How can I do that in LabVIEW? How can I check when my square wave starts looking like a triangular wave? please help

[ifyo]

gud evening all

I have an op amp circuit built on the NI ELVIS board and I input a square wave from function generator to the circuit. I have to determine the slew rate of the op amp.In the labs, one would run the square wave over a range of frequencies and stop when the square wave looks like a triangular wave. At that frequency, one would then measure the slope of the signal and that will give you the slew rate. How can I do that in LabVIEW? How can I check when my square wave starts looking like a triangular wave? please help

[johnsold]

The slew rate does not depend on the input frequency. The triangle method is just one way of finding the slope.

In the lab with the oscilloscope if you trigger on the rising edge of the input square wave at low frequency and look at just the rising (or falling for an inverting amplifier) part of the output you should see the same slope as you get with the triangle method. Set the sweep to ~ 1 us/div.

WIth LV you would do the same thing.  Trigger at the appropriate time and sample fast enough that you get multiple samples during the sloping part of the output. Measure slope.

If you want to us the triangle method in LV, look for the frequency where the output amplitude starts to drop.  The triangle point should be just below that frequency.

Lynn

[ifyo]

thank johnsold for reply, my problem is how can i use the built in lab view vi to do the frequency inreament to get the level where the square wave will become triangular.

[johnsold]

This sounds like a school assignment.  We will help you learn about LV but will not do your work for you.

What have you tried?  What works? What does not work?  In what way does it not work? Please post what you have tried with typical data saved as default.

Can you generate a square wave? Can you measure the amplifier output? Can you repeat the process?

Lynn

[ifyo]

thank you johnsold

i designed a voltage follower using 741, on an Ni elvis instrument. i can manually do the slew rate experiment by varying the frequency range on the coasre frequency knob( using a square wave) of the function generator until the slew rate occurs at 0.5volts, my problem is how can i do the same experiment with built in vi in labview with out touching the knob on the function generator and once the triangular wave starts to show the lab view program will automatically stop.

find attache my vi

[johnsold]

I do not know what hardware capabilities are in ELVIS.  If it has an analog output or a digital output, you will need to use one of those to generate the square wave rather than the oscillator on the board.  Verify that the AO slew rate is much faster than the expected slew rate of your op amp or you will need to use some other method to generate the square wave.

Put a while loop around your Read VI so that you can get more than one set of data. Inside that loop you will also write to the AO or DO which generates the square wave and you will test the data and change the frequency if necessary.  A state machine architecture would be a good starting point.

Lynn

[ifyo]

thanks johnsol for your reply, the Ni elvis instrument has 16 analog input and two anolog out put (dca0 and dca1), and the pin 3 from the 741 is connected to Dca0 analog output  for the generation of the square wave, my confusion is how can i vary the frequency on the function generator from 50hz to 100khz to get the slew rate reading

[NIquist]

Keep in mind that although the 741 is old and relatively slow by modern standards you'll still need to sample faster than 100KHz to accurately measure its slew rate.  Can your hardware sample at 1MHz?

[johnsold]

NIquist,

He is not sampling at that rate.  He is feeding a square wave of that frequency to the input of the op amp and expecting to see a triangle wave (slew rate limited) at the output.

ifyo,

I have never used ELVIS, so I do not know how to set the outputs.  The VI you posted did not have any Analog Out subVIs.  I think you need to use the AO and not the built in function generator.  Read the ELVIS Manual.

Lynn

[ifyo]

HI  johnsold thanks for you reply,

   i can actually generate  the square wave signal using the signal generator vi connected to the daq, my problem is how can i increase the steps of the frequency on the signal generator automatically,  so as to get a traingular wave. or once the input from the DAQ anlog AI is feed to the derivative VI and the output is equal to 0.5v, the lab view  stop executing automatically.

find attached the VI i have been able to put to gether