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I'm testing my peak hold circuit with both LTspice and real one.

When inputting a pulse of 500 mV, LTspice gives 800 mV (green one in upper left) as peak hold. However, real circuit, which is designed based on LTspice, gives 500 mV when inputting a pulse of 500 mV (green one in lower left: oscilloscope.)

Which one is proper?

In fact, when looking at the very beginning of the green one in the real circuit, there is overshoot, which is close to 600〜700 mV. LTspice is demonstrating this overshoot and holding this peak properly?

Do anyone know the reason?

enter image description here

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    \$\begingroup\$ The 800mV can not be physical. You should have the same voltage at "before" as at the input. Something is wrong in the simulation. I would always believe the measurement, especially if it matches the expected theory. \$\endgroup\$ Nov 20, 2020 at 16:24
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    \$\begingroup\$ The overshoot at "After" is due to the capacitive coupling from the gate to the source of U16. 3.3V voltage change is pushed to the source. The 100nF swallows that extra charge once the MOS turns on. \$\endgroup\$ Nov 20, 2020 at 16:28

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You need to use some sort of models for the diode and transistor, otherwise their values default to zero, mostly. For example, the default diode is ideal, and has 0 V forward drop, while the transistor has zero resistances and capacitances. At the very least, when trying to verify a circuit, you have to choose a model, even if it's not exactly what you have, or need. Even if you do have the models you need, they might not be well made and thus, prone to giving unreliable results. Also, unless there's some series resistance involved, or you use the current through it, adding a capacitor across a voltage source is useless.

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  • \$\begingroup\$ Thank you very much, I try to test it following your suggestions! \$\endgroup\$
    – user268029
    Nov 20, 2020 at 20:45
  • \$\begingroup\$ You may also want to use a PMOS instead of U16. \$\endgroup\$ Nov 20, 2020 at 21:53
  • \$\begingroup\$ After setting the "series resistance" of my voltage source to 1K, "Before" pulse gets close to 500 mV (updated my pic). Probably, I have to match Input/Output impedance b/w my voltage source and the first OpeAmp in LTSpice. .......... But, in the real circuit, I'm using a function generator of output impedance of 50 ohm. "series resistance" and output impedance are different ?? \$\endgroup\$
    – user268029
    Nov 21, 2020 at 7:19
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    \$\begingroup\$ Dear a concerned citizen.... I also found that a universal circuit board and a breadboard give different pulse hight! Pulse hight in the universal board gives ~ 800 mV with my pulse generator of 50 ohm input impedance, which is almost same with one I got by LTSpice (where series resistance is also set to be 50ohm)..... I was surprised that the observation change depending on the universal board and breadboard. So, I will close this question, thank you ! \$\endgroup\$
    – user268029
    Nov 21, 2020 at 9:21
  • \$\begingroup\$ The generator can be modelled as a voltage source with a series resistance, so 50 should do. But in your schematic you also have a 1k resistance to ground, at the noninverting input, which means now you have a resistive divider. Do you have that on the breadboard? I was about to post this, when your comment came in. Whenever a breaboard is involved, don't forget that there are wires and various contacts that can interfere, sometimes with dramatic results. If this answer solves your problems, mark it down, so that future searches will show a question with a selected answer. \$\endgroup\$ Nov 21, 2020 at 9:39

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