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I'm having trouble with simulating a Sallen-Key single-supply low pass filter using LTSpice. After fiddling around with the circuit for an hour I'm hoping for some assistance in troubleshooting.

For a transient analysis I'm inputting a 1 Khz 1 V signal and apply a DC offset. The gain should be 1 at this frequency. But at the output of my filter I'm getting a very very small amplitude centered at the DC offset.

Trans Simulation

For an AC simulation I'm inputting a small-signal AC amplitude of 1V. Again, I see a very small gain, and this time it's shown to be across all frequencies.

AC Simulation

Based on the circuit design, these results are not what I would expect. I think my error must be in how I am implementing it using LTSpice, which I'm not an expert in using. I'd appreciate any input on what I might be doing wrong.

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2 Answers 2

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The LT1028 is intended for operation from +/- 15 volts, and is only speced as low as +/- 5 volts, and you are running it at 3.3 single-ended. Try changing V2 to 30 volts.

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  • \$\begingroup\$ My design will require a low voltage. I'll change to a different op-amp. \$\endgroup\$
    – JeremiahOgletree
    Commented Mar 17, 2016 at 22:59
  • \$\begingroup\$ Nevermind the previous comment. \$\endgroup\$
    – JeremiahOgletree
    Commented Mar 17, 2016 at 23:51
  • \$\begingroup\$ @JeremiahOgletree - try the 30 volts anyways, and see what that does to your simulation. That way you'll be able to examine your filter component values. \$\endgroup\$
    – WhatRoughBeast
    Commented Mar 18, 2016 at 2:10
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  1. At least +6V/-6V(-4V) power supply is required in this particular case (Vin_magnitude=1Vac + the dc offset inserted by R3/R4).
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