slew rate determination

[emeho]

hi all,

i am new to lab view programming, and i want to use lab view to do the slew rate experiment using 741 op amp,

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? Currently I am using Analog Input vi's to capture my waveform data and display it on the waveform graph (Measurement I/O -> Data Acquisition -> Analog Input). Please help, this is urgent.

[SteveChandler]

You can use the Transition Measurements VI in the Waveform Measurements VIs palette.

[RTSLVU]

You can use one of these formulas after measuring the peak to peak voltage and rise time

Slew Rate = 0.6*Vpk-pk/rise time (using 20%-80% rise time)

Slew Rate = 0.8*Vpk-pk/rise time (using 10%-90% rise time)

[emeho]

thank u SteveChandler and RTSLVU,

    i have written  a lab view code for the slew, which is attached to this , how will i tell lab view to stop automating the process once the wave form is triangular and it gives slew rate =o.5.

    how do i integrate a vi that will tell me when the wave is triangualr and is equal to 0.5 which is the manufacturers value for 741 op amp slew

[emeho]

thank u SteveChandler and RTSLVU,

    i have written  a lab view code for the slew, which is attached to this , how will i tell lab view to stop automating the process once the wave form is triangular and it gives slew rate =o.5.

    how do i integrate a vi that will tell me when the wave is triangualr and is equal to 0.5 which is the manufacturers value for 741 op amp slew

Attachments:

[emeho]

thank u SteveChandler and RTSLVU,

    i have written  a lab view code for the slew, which is attached to this , how will i tell lab view to stop automating the process once the wave form is triangular and it gives slew rate =o.5.

    how do i integrate a vi that will tell me when the wave is triangualr and is equal to 0.5 which is the manufacturers value for 741 op amp slew

[johnsold]

The technique you describe (and which you or one of your fellow students asked about not long ago) seems fine for use with an uncalibrated oscilloscope but a rather overly complicated and not very efficient way to measure the slew rate.  Are you doing it that way because you are required to do so, because it is a way to learn to program several techniques in LV, or some other reason?

Your program does not make much sense.

1. Why use of a shift register to pass the start time when it is never changed inside the loop.  A tunnel will do.  (Although I suspect that you may actually want to change it, in which case you will need the shift register.)

2. The TIME STEP STEP control does not actually change the time steps.  What is this supposed to do?

3. You do not know whether the Function Generator or the DAQ Assistant will execute first.  If this matters, you need to create a data dependency, probably by wiring the error out of one to the error in of the other.

4. Controls which will not change while the program is running should generally be placed outside the loop.  For your program, this is likely all of them.

5. A stop button or some error checking to make suer the program will stop in a reasonable time is probably a good idea.  What if someone sets MAXIMUM FREQUENCY to 10^300?

6. You do not need to call Tick Count twice inside the loop.  Just wire the Millisecond Timer Value indicator to the Tick Count function above the IME STEP STEP terminal.

Lynn

[emeho]

 thanks john sold johnsold

 1. am using the shift register to change the function generator frequency from one step to the other ( eg 100hz to 1MHZ)

2.THE TIME STEP HELPS THE WAVE FORM TO SETTLE BEFORE THE FUNCTION GENERATOR FREQUENCY GOES TO THE NEXT FREQUENCY

3. THE FUNCTION GENERATOR EXCUTES FIRST BEFORE THE DAQ

 WHAT I NEED IS TO KNOW HOW I CAN DTERMINE WITH Lab view that the wav e is triangular and is 0.5 which is the slew rate for 741 op amp. at the moment the vi is working when i view it with an oscilloscope, is just that i want to be able to integrate a vi into the program i have written that will help me measure the wave and tell that is 0.5

[samthespa]

These formulas worked for me.  Thanks guys 🙂

[emeho]

thanks samthespa

  could you pls tell me how you used the formulas and they worked for you