Problem with measuring the Slew Rate of an LM741CH Op-Amp on Multisim

Discussion in 'Homework Help' started by ARRozental, Oct 22, 2012.

  1. ARRozental

    Thread Starter New Member

    Jul 17, 2012
    4
    0
    Hello all
    I am attempting to complete an assignment, which asks for a measurement of the Slew Rate of the LM741 and LF356 Operational Amplifiers. This is being facilitated through the use of Multism v11, in which I have constructed a Unity Gain circuit with an LM741CH, which is the Op-Amp that I wish to focus on for the purposes of this question.
    The input signal is a 2V square wave. I have tried to make Slew Rate calculations based off the signal displayed by the Multisim virtual oscilliscope, at many different input frequencies (10Hz to 10MHz), but I seem to be getting the wrong answer consistently.
    The National Instruments Datasheet concerning the LM741CH, specifies a typical slew rate of 0.5V/us, from a Unity Gain configuration. I am assuming that this reading can be obtained without placing a particular kind of load at the Op-Amp's output, as the Datasheet does not specify one.
    Now, as an example- the Input Voltage is 2V, and the Input Frequency is 100KHz. Observing the Output Square Wave, some Slewing is clearly evident. So, using the Oscilliscope, I determine that deltaV is 2V, and deltaT is 701.493nS. Thus the Slew Rate can be calculated thus:

    SR = deltaV / deltaT
    = 2 / 701.493E-9
    = 2851061.949
    = 2.8V/us

    The result above is of course a substantial deviation from the Datasheet's typical value, and this error seems to be translated accross the whole range of frequencies and input voltages. I realise that it is customary to measure the slew rate between 10% and 90% of the Rise or Fall Time, but I would like to get a Slew Rate Value at least approaching the typical one before doing that.
    I am at my wit's end, and I was wondering weather anyone on this fantastic Electronics Resource might be able to help me work out where I am going wrong.

    The attachment contains an image of the circuit and waveform for the calculation above.

    Looking forward to hearing some suggestions.
    Many Thanks.
    Adam.
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    Didn't your school give you a specific lab to do with known components?

    Make your circuit with a 1k resistor as the Ri and a 10k resistor as the Rf and set the signal source to 40 kHz @ 2 V pk to pk. The output signal should resemble a triangle wave. You will see appoximately 0.5V per microsecond slew rate.
     
    Last edited: Oct 22, 2012
  3. ARRozental

    Thread Starter New Member

    Jul 17, 2012
    4
    0
    Thanks for the reply JoeJester
    Unfortunately, my assignment does not specify any specific components to be included in the circuit with the amplifier. Only the Amplifier types are specified- LM741 and LF356. The assignment simply states: 'Measure the Slew Rate for different op-amps (LM 741 & LF 356) and compare with manufacturer's data sheet'. The datasheet for both Amplifiers (I chose the C type of both amplifiers) lists the nominal Slew Rate, when each Amplifier is connected in Unity Gain mode.
    I rebuilt the LM741C's circuit with the Resistors connected as you suggested, JoeJester, and obtained improved, but still puzzling results- please see the circuit and in/out waveforms in the attachment. Also, a 40KHz input frequency did not result in a Triangle Wave output- as can be seen in the attachment. A Triangle Wave is output when the frequency passes 1MHz, however this attenuates the Output Voltage as the Amplifier struggles to react to the changing square wave input. Measuring the Slew Rate at this point also results in a similar result- currently I am getting about 2.8V/uS at 40KHz, and about 8.7V/uS above 1MHz input frequency.

    SR = deltaV/deltaT
    = 2 / 705.224E-9
    = 2835978.356
    = 2.8V/uS

    I am still unable to understand why Multisim 11 is generating these results, that are so contrary to those specified on the datasheet. Placing Rin and Rf in the circuit did improve the results, thus I can try altering the ratio between the two to obtain a good slew rate value- however, if the Datasheet specifies 0.5V/us in Unity Gain Mode, then I would have thought that the Simulation Software would at least somewhat adhere to it.
    Anyway, thanks for the help thus far- any further thoughts that anyone may have regarding this problem would be greatly appreciated.

    Thanks
    Adam
     
  4. ARRozental

    Thread Starter New Member

    Jul 17, 2012
    4
    0
    P.S- I will also try conencting the Amplifier in Inverting and Non Inverting mode, to see which results issue forth.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Again, Multisim has problems reading datasheets.

    The datasheet of the 741 opamp shows it having slew rate limiting typically at 40kHz with more than only plus or minus 3.5V peak.
     
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    I gave those values from the lab below.
     
    Last edited: Oct 24, 2012
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    Even at 1 kHz, the slew rate was the same as 40 kHz with my simulator.
     
Loading...