I created an LTspice model of my circuit which consists of an op-amp in a unity gain voltage follower configuration which feeds a SAR ADC. I also included the flywheel circuit in front of the ADC. There are two models, each one having a different op-amp.

The one labeled "bad" uses a TI 4111 which isn't really designed to handle the capacitive load my circuit has. I originally designed with this op-amp before I knew the capacitance values. Thus I made a second circuit that uses an op-amp with unlimited capacitance drive.

Using the analog suggestion on how to measure phase margin I plotted the following.

Not sure how to interpret these, it seems that the TLV4111 has more phase margin than the analog ADA4807 which seems backwards to me given the load circuit. Any suggestions?

enter image description here

Updated the picture with the suggestion of adding a DC source on the + input; it doesn't seem to have made a difference:

enter image description here

  • \$\begingroup\$ Regarding sources V1, V2 and V4 and the use of the switch in AC analysis: do read my answer in How to implement AC analysis in a resonant circuit in LTspice. \$\endgroup\$
    – Huisman
    Commented Oct 2, 2019 at 9:12
  • \$\begingroup\$ What are you actually trying to test? Why are you injecting noise in the feedback loop of the opamp? This way you're not simulation "an op amp in a unity gain voltage follower configuration". \$\endgroup\$
    – Huisman
    Commented Oct 2, 2019 at 9:24
  • \$\begingroup\$ I'm digitize an old analog vehicle sensor that provides a voltage to the cars ecu. Recently I realized that the ecu has front end circuitry that results in a large capacitive load to my circuit. it was obvious from the op amp datasheet I was using would be unable to drive this load. I want to analyze this. Here is some details on my design. e2e.ti.com/support/amplifiers/f/14/p/839923/3121607#3121607. Ignore the current draw of 100 mA turns out that was a bad measurement, the output draws wmA ish. \$\endgroup\$ Commented Oct 2, 2019 at 11:00
  • \$\begingroup\$ Regarding injecting noise, I'm not sure what you mean. I setup the model per this analog video. google.com/… \$\endgroup\$ Commented Oct 2, 2019 at 11:11
  • \$\begingroup\$ Link doesn't work... \$\endgroup\$
    – Huisman
    Commented Oct 2, 2019 at 11:18

1 Answer 1


Read a thread on Here that said since I'm powering the op amp 5vdc that I needed to bump the voltage input to be in between the rails.

Citation from that answer:

As mentioned by just about everyone in the comments, your op amp doesn't work because your DC bias point is 0V and the op amp will only work for inputs between its rails (0V and 5V).

Because you use the opamp in a unity gain voltage follower configuration, the input of your opamp is the positive input terminal of the opamp.
To prevent the operating point running into the power rails, you should not short this positive input terminal of the opamp to GND, because that causes an operation point of 0V. Instead, insert a voltage source (e.g.) 2.5Vdc on it (source V5 with value {Vin} as shown below). The AC source should have an DC component that is 0Vdc.

You can verify this by stepping the voltage of this DC voltage source V5 with .step param Vin LIST 1V 2V 3V : they should yield the same outputs for AC transient analysis.

enter image description here

  • \$\begingroup\$ You are indeed correct. Sorry about that. \$\endgroup\$
    – efox29
    Commented Oct 4, 2019 at 16:09
  • \$\begingroup\$ OP is doing a AC transient while the operation point is clipping. Maybe I should start with that... and I'll attach an LTSpice scteenshot showing where to add this DC source (when on PC again) \$\endgroup\$
    – Huisman
    Commented Oct 4, 2019 at 16:16
  • \$\begingroup\$ Did the update, see the original post... Didn't seem to make a difference, did i miss something? \$\endgroup\$ Commented Oct 6, 2019 at 1:52

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