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LTspice simulation results don't match breadboarded AD8001 circuit

Category: Software
Product Number: LTSpice

I built the function generator circuit on a breadboard. I applied a signal with an amplitude of 1.06Vpp and a frequency of 5MHz using the DDS 9851 and then sent it to the operational amplifier, AD 8001. After amplification, the signal increased to 4Vpp while maintaining the same 5MHz frequency. However, some distortion appeared in the signal after amplification. To address this, I connected a ferrite bead with a rating of 1kΩ in series with the signal. After using the ferrite bead, the signal became much cleaner with very little noise. This allowed me to remove the large laboratory function generator and apply the amplified signal directly to my sensor.

This is my first experience working with LTSpice software. I recreated the same circuit in LTSpice, using the same resistor values and one simulation with more resistors value. However, in the simulation, the signal amplitude did not increase as it did in my real circuit. The signal did not amplify after using the same operational amplifier in LTSpice. Do you think LTSpice should amplify the signal as much as the real circuit does when using a single operational amplifier?

I have drawn a picture of my real circuit, which I will show to you along with the LTSpice circuit and its results. Additionally, I will show you the signal results that I measured with an oscilloscope. Could anyone please share your experience and tell me whether this significant difference in signal amplitude between the real circuit and the LTSpice simulation is common when using simulation software? Or could this discrepancy be due to a mistake on my part, given that there are differences between reality and software simulations? I f there is some mistake on my side, can someone highlight my mistake please



Modified title; added AD8001 keyword
[edited by: MStokowski at 7:02 PM (GMT -4) on 20 Aug 2024]
  • Hi  ,

    It would help us if you uploaded your ASC files. There is no guarantee that we can reproduce your circuit verbatim from a photo. Simply drag your circuit (.asc) file to the reply dialog. I don't see the scope results.

    mike

  • Hi  ,

    I tried simulating the part you are using and followed your circuit as shown in your screenshot.

    I was able to get your expected results.

    In the screenshot above, I was able to amplify the green colored waveform (Which is the input) into an amplified version of it (The blue signal).

    It is best if you could share with us your simulation file so we could check on our side what went wrong with your simulation.

    For your reference, I have attached here my version of the simulation that I think best matches your expected results.

    for context here is what I did:

    1. I downloaded the SPICE model of the AD8001A from the ANALOG.com website.

    2. created a symbol for this part. (I did not create a similar op-amp symbol just to hasten the process)

    3. Followed your schematic and viola!

    Attached file is the zip file which contains, the symbol, simulation file and model file.

    Best Regards,

    Earl

    AD8001A.zip

  • I have attached the circuit file in the comments, dear sir. I have two question.
    I made the AD 8001 symbol and used in the circuit

    1)  I generated two signals: one is 2 V peak (Vp) and the second is 2.5 V peak (Vp), with frequencies of 5 MHz and 6 MHz, respectively. In the real circuit, if I don't use ferrite beads after the operational amplifier, both signals show a very high level of distortion. I managed to reduce the noise and distortion by using ferrite beads in the real circuit on bread board. However, in the LTSpice simulation, when I don't use ferrite beads and the low-pass filter (LPF), the signal amplitudes become very accurate. But when I use either the ferrite beads or the LPF (or both), there are fluctuations in the signal amplitude. For example, if I only consider the 2.5 Vp signal and check its amplitude throughout, in one cycle, the amplitude sometimes drops to 2.44 Vp, 2.48 Vp, or 2.49 Vp. How can I achieve a constant amplitude of 2.5 Vp? Why does adding the LPF and ferrite beads in the simulation cause amplitude fluctuations, whereas in reality, they improve the signal's distortion and noise levels?

    2)  When I use the LPF before the operational amplifier, there is a slight drop in signal amplitude, but I understand this is a common occurrence. Could you please check my circuit and suggest any improvements or modifications?
       functional generator project complete.asc

  • Hi  ,

    I have not yet checked the Simulation file you shared but here are some of my few thoughts with regards to your inquiry.

    1. There could be numerous reasons why this occurs.

    • The noise introduced by the Op-Amp used is not accurately modeled in the Op-Amp model you are trying to use.
    • There could be discrepancies on the model of the Ferrite bead modeled used in your simulation

    2. Is this related to your simulation or with the actual circuit you are bulding?

    Best Regards,

    Earl

  • Dear Sir, I am responding to your question:
    Question: Is this related to your simulation or to the actual circuit you are building?
    Answer: This is related to my simulation, based on which I will reconstruct the circuit on a breadboard again, i will do changes in my previous real circuit that i built on the breadboard. The results are now quite good because I used an RLC circuit to remove distortion and noise. I achieved 2.5Vp and 2Vp, respectively. Another important point is that I achieved this signal magnitude with low resistance values in the feedback, as the AD8001 datasheet recommends using resistors in the ohm range. 
    If you could check my circuit and suggest any modifications if you find flaws, I would be highly appreciative, Sir.
    I determined the values of the RLC circuit components using an RLC calculator.
    3426.functional generator project complete.ascI
     am attaching my LTSpice simulation file for your reference.

  • Hi  , 

    Your simulation seems fine Slight smile

    I checked it and did not find anything that would cause error in the simulation.

    Thank you for using LTspice. Slight smile

    Best Regards,

    Earl