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some of you probably remember my last question, but I like to reiterate: I created a bandpass filter with the help of the online tool FilterWizard, which should block below 30 kHz and above 300 kHz; the whole thing with ± 9V supply voltage and 3dB gain in the pass band. In addition, it has been optimized in the direction of Low Noise.

Under the ‘Voltage Range’ tab on the website, it is stated that the signal may range from -8.25V to 8.75V.

As you can see in the schematic created by the online tool, there is an AD8022. Since I use PSpice for TI 2021.1, I “only” have the components from Texas Instruments. I searched back and forth for a long time until I found a (apparently) suitable operational amplifier. I entered the specifications of the AD8022 (slew rate, gain bandwidth, Voltage Offset, Voltage Noise) on the Texas Instruments website and found and installed the THS4021. I've also tried others before. The problem is that no matter how good the op amps are on paper, the simulation always results in nonsense. I have already cross-checked whether I made a mistake in the schematic. What's going on?

This project is for training purposes only – I am not a qualified electrical engineer. Unfortunately I cannot offer you an .asc file.

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Analog Filter Wizard – planned Magnitude

Analog Filter Wizard – planned Magnitude

Analog Filter Wizard – given circuit

Analog Filter Wizard – given circuit

Analog Filter Wizard – Voltage Range tab

Analog Filter Wizard – Voltage Range tab

my circuit (jointed image)

my circuit (jointed)

Simulation: red curve: Voltage Source; green curve: VOut

Simulation

https://www.analog.com/en/products/ad8022.html#product-overview

https://www.ti.com/lit/ds/symlink/ths4021.pdf?ts=1635693680345

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Edit: Attached is a picture with graphs of what components R3, R5 and C7 see at their input.

R3 R5 C7

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Edit 2: While everything is ok on the left side of the C7, ‘Node N02220’ (orange line) sees this… .

sub-image of the circuit

what the node sees

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Edit 3: I have now reduced the voltage to a tenth. This seems to work to me, except that from around 250kHz the amplitude of VOut is no longer as large as it should be up to 300kHz.

enter image description here

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    \$\begingroup\$ perhaps the negative rail psu v3 is wrong polarity? \$\endgroup\$
    – Marko Buršič
    Nov 1, 2021 at 13:59
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    \$\begingroup\$ I have got the impression that your supply voltages are not correct. Please check. \$\endgroup\$
    – LvW
    Nov 1, 2021 at 14:05
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    \$\begingroup\$ Investigate each stage's output waveform from L to R until you find the one that's clipping. \$\endgroup\$
    – user16324
    Nov 1, 2021 at 14:05
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    \$\begingroup\$ +1 for annotating DC biasing voltages in the schematic ! 1) Try reducing the amplitude at the input, how does 85 mV (100x less than you have now) work? 2) Those THS4021 opamps are bloody fast, hmm, are they unity gain stable? Let's look in the datasheet: Hmm: "Stable at a Gain of 10 (–9) or Greater" I think that's your problem! \$\endgroup\$
    – Bimpelrekkie
    Nov 1, 2021 at 14:38
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    \$\begingroup\$ I don't think you need such a high GBW product. My guess is that Analog Devices suggests the AD8022 because they sell it and it should always be fast enough. But you're not making much gain and your BW is also limited at 300 kHz so a GBW of 10 MHz might suffice. I would try the NE5532 which is a well known opamp so it should have a model. Just ignore slewrate and noise (although NE5532 is "low noise") for the moment. \$\endgroup\$
    – Bimpelrekkie
    Nov 1, 2021 at 14:59

2 Answers 2

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See this in the data sheet, front page: -

enter image description here

These devices are not suitable connected as unity gain amplifiers.

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Summary for future readers: After learning the word "Unity Gain" in the sense of operational amplifiers, I looked for such opamps and found the OPA820. The early attenuation above 270kHz (instead of the desired 300) is probably due to a reading error from me. I misunderstood the gray chart in the online tool. Where that vertical gray line is on the chart, it is not 300kHz. Anyway – it was just a matter of practice. Thanks to you all. 🤝

PS: There is no proper German word for unity gain. Well ... Whatever.. enter image description here

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