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Well I'm struggling today, maybe I'm putting too much thought into this. I'm trying to design the proper virtual gnd circuit for my opamp filter. In real life my output will be a DAC with a current source into a 75 Ohm resistor. The resulting voltage will always be positive since the current source is always positive.

So anyway I wanted to do an AC simulation in spice to make sure my changes to make this filter a single supply would work. I don't quite understand why the simulation only matches the dual supply one if I put a 2Vdc offset on the AC source, 1Vdc offset results in much more attenuation at high frequencies.

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The other thing I struggle with is I don't even know if this is the right way to model my source. I feel like putting this strong DC offset in the AC source is throwing off my understanding of the virtual gnd circuit.

I've been trying different things and went back read about how AC simulations work, but still feeling frustrated by not understanding everything here.

Does anyone have any insight on why it only works at 2V offest, and if there is a better way to model my input?

The opamp I'm using is the Analog devices AD8051

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  • \$\begingroup\$ If your virtual ground is formed from a current DAC and 75 ohm resistor, why are you modeling it with a voltage source and resistor divider? Better to model it with a current source and 75 ohm resistor. \$\endgroup\$ – The Photon Mar 4 '15 at 21:26
  • \$\begingroup\$ My source, not my VGND is a current dac and a 75 Ohm resistor. But yeah I have tried that I was a little confused though. Since the opamp is single voltage don't I need to offset the AC source somehow so it doesn't go negative? \$\endgroup\$ – confused Mar 4 '15 at 21:48
  • \$\begingroup\$ What part number is your op-amp? (A link in your question to the datasheet would help us help you) \$\endgroup\$ – The Photon Mar 4 '15 at 22:52
  • \$\begingroup\$ Ok just added the link \$\endgroup\$ – confused Mar 4 '15 at 23:21
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The basic problem is that your half rail generator is being dragged down to less than 2.5 volts by the dc offset on your source voltage. Then you have what appears to be a gain of 2 opamp circuit but the interactions of the source voltage superimposed on what is meant to be a steady 2.5 volts is screwing things up. Replace the two resistors of 100 ohm with a fixed dc voltage source is my advice.

By the way, with a gain of nearly 2 the opamp dc output voltage will be naturally centred a little lower than 4 volts and this, I believe is not what you want.

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  • \$\begingroup\$ Thanks @Andy aka the gain comment made me realize why it only works properly with a 2.5V offset, I must be clipping later on. \$\endgroup\$ – confused Mar 5 '15 at 4:27

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