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I am trying to use the AD8067 in LTspice (v17.0.33.0). The model for this opamp ships with LTspice.

schematic

I have rigged it up as a follower, and I am seeing 150pA input current. Per the datasheet it should be <0.6pA.

  • I tried playing with the SPICE settings. As you can see in the screenshot, I reduced GMIN and ABSTOL (and I tried a bunch of others...).

  • As far as I can tell from the .lib file, the model does implement bias currents correctly. Also, a comment in the model file says: Tested on MultSIm, SiMetrix(NGSpice), PSpice. NB: not LTspice.

I suspect it is a numeric issue with LTspice. Is there another configuration option I need to change?

polwel
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  • Nevermind the deleted answer, my eyes are playing tricks on me: I saw +/-5 V, not +5 V. At any rate, there's no need for the added `.options`, and having a voltage greater than zero at the input will mean you're measuring both the bias and the polarization current. You need to set `V2` to zero, add a gain of 10 with a feedback and load of 1 kOhm (I omitted the gain). It does look like the unipolar supply doesn't play nice with the model; it has nothing to do with numeric precision. – a concerned citizen Apr 07 '22 at 12:32
  • Probably not your issue, but AD8067 is not guaranteed stable for gain < 8, so a voltage follower is not a stable configuration. – Spehro Pefhany Apr 07 '22 at 12:59
  • Huh. Yes, the model indeed seems to produce sensible bias currents with a +/-5 V supply, but not for assymetrical supplies (e.g. -5/+10). Weird. @aconcernedcitizen Are you just saying the model is not correct? – polwel Apr 07 '22 at 13:19
  • @SpehroPefhany Yes, thanks for pointing that out. Forgot about that when I set up the minimal example. Prospective application would have been an integrator, so not an issue. – polwel Apr 07 '22 at 13:22
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    @polwel You could write at the address in the *Help > About* to send a bug report, if you're so inclined. But be sure to check again with the test setup as depicted in the datasheet (1k + 110 feedback, 49.9 + input, 1k load). I did and got nA current. If you'll get the same, it's a reason for concern. It certainly looks like the model doesn't account for unipolar supply. – a concerned citizen Apr 07 '22 at 13:52
  • I reached out to the LTspice team. – polwel Apr 07 '22 at 14:45
  • @polwel An integrator configuration is not necessarily stable with this op-amp. – Spehro Pefhany Apr 07 '22 at 15:26
  • Agreed. Wasn't thinking earlier. It's stabilized by input capacitance. – polwel Apr 07 '22 at 16:44

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