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I have the following circuit in Multisim and am getting the Bode plot at the bottom. Could someone please explain to me what is happening?

The AD8651 is listed as having a 50MHz bandwidth. Why is it that it goes astray at 17.4kHz?? I think it might be something wrong with my design but I am not quite experienced enough at this to narrow it down.

enter image description here

PeterJ
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user1003131
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1 Answers1

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I'm not familiar with multisim, but from the look of things, you're driving a set of op-amps powered with 0-3.3V with a 1V P-P sine-wave centered around 0V (which means it swings from +0.5V to -0.5V).

As such, your op-amps will be railed for 1/2 of the input cycle, which will make your filter behave very strangely. I suspect the fact that it looks like it's working at all may be an issue with the simulation not being very accurate, or behaving strangely when used outside of the op-amp's "proper" bounds of operation.

You need to add some DC bias to your input, so it idles at \$ \frac{Vpp-Vss}{2}\$.

Connor Wolf
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  • Thanks for the reply Connor. I added the offset and it pushed back that significant gain in attenuation to about 100 kHz. The shape hasn't changed, it still seems like a weird shape to me. – user1003131 Oct 29 '13 at 10:46