Sweep isn't working correctly, but I don't see why

[Euler's_Identity]

It seems like a simple thing but something's wrong. 

See the attached circuit.  It is a 10k potentiometer across a 10v dc source.

So that I could name the node, such that it'd be easy to spot to set up the analysis output variable, I took the wiper of a pot that is across the 10v dc source into the very high impedance of a follower op amp (1/4 of a TL084).  Then I swept the resistance to gnd within a dc operating point analysis and used the dc point at the input of the op amp as the output variable.  Hence, as I'm seeing things, I should have had the two variables necessary to plot x vs y in an Excel xy scatter plot.

But that's when things went wrong.  The op amp is running from a +/-15v split supply, and the model used includes the supply lines.  Therefore, I should have, at least I thought, seen the voltage swing from near zero to 10v, but that's not what I got.  Instead, I got zero to half of whatever dc source I chose??

Any ideas what went wrong?

[Euler's_Identity]

Incidentally, using the output of the follower as the output variable, I still get the same thing.  I'd say that's good evidence that it's the sweep that's at fault and not something going wrong with the op amp.

It's like something is causing the dc source to get halved.

My next test will be sub'ing the opamp with an impossibly high value resistance.

[Euler's_Identity]

This plot thickens.

See new attached. 

Yes, I have more evidence that the sweep is wrong.  I can see that the op amp produces the expected 10v, when given what would be the input from the pot circuit were the wiper at 100%, but the wiper voltage still taps out at 1/2 the dc source voltage. ??????

Just for the heck of it, I temporarily popped in another 10k resistor above the pot.  With this setup one would expect an output voltage that taps out at 5v.  I frankly don't understand the results I got.  Then I moved the resistor under the pot.  Once again, one would expect the voltage to tap out at the supply, but, instead, I got what looked like 2/3rds the supply voltage. ???  What the?!

This so bizarre...

Am I reading something wrong?  Am I accidentally running my results through an unexpected function before the data's had? 

Am I misinterpreting the sweep?  What the heck could I be doing wrong? 

Message Edited by Euler's Identity on 09-04-2008 09:36 PM

[Euler's_Identity]

Ok, I've got something now.

See new attached.

I replaced the dc source with the equivalent dc current source (10v/10k = 1mA), and now the voltage sweep taps out 10v.

Wow but that was weird!

Oh well, at least I have a solution now.

Thanks for the help!  ...that was a joke 🙂