An opamp no longer has the characteristics of an opamp when power is removed
from it. The internal diodes and resistors which are pointed to the V+ rail are
now pulling the + input somewhere towards ground and loading anything on the
input to ground. It looks like your circuit is behaving quite normally.
Rick

Donovan Parks wrote:

> Hello,
>
> When I connect V+ and V- both to ground to effectively turn off the op. amp,
> the voltage between the resistors in the voltage divider is 1.3V (measured
> with an X10 probe and paid more attention this time).  I am indeed using a
> R-R op. amp that can handle inputs close to both the positive and negative
> rails.  The reference voltage is connected to the non-inverting input on the
> amplifier.
>
> What I can't figure out is why the voltage between the resistors in the
> voltage divider is 1.3V (I when the op. amp has V+ and V- are ground.
>
>              200k    200k
>  +5V --/\/\/\----/\/\/\---ground
>                      |
>                      |         100k
>                      ------/\/\/\------ non-inverting input of op. amp.
>
> I would expect the voltage between the two 200k resistors to be 2.5V as the
> op. amp should draw almost no current, but instead it is at 1.3V.  Setting
> V+ and V- to ground has somehow caused the op. amp to draw more current (I
> have measured this the current into the op. amp to be 11uA which accounts
> for the lost voltage).  Why?
>
> Even more interesting, is that when I have V+ at 5V and V- at ground.  If I
> turn the +5V supply off and then on, the current being drawn by the op. amp
> decreases from around 11uA when I first turn on the power to 0uA (less than
> my DMM can measure) in about 30sec.  Does an op. amp. have a "warm up" time?
> Is the results I'm seeing expected?  How is this "warm up" time connected
> with my voltage divider.
>
> Note: that the problem is easily solved by replacing the 200K resistors with
> 25K resistors, but I wish to understand why this is happening.
>
> Regards,
> Donovan Parks
>
> > I'm not sure from your description where the 1.7 volt is I thought the
> > ref was connected to V= or V- But if you expect to use an an op amp on a
> > single supply and have inputs close to ground you must use an op amp
> > who's input range common mode goes to ground most of them don't check
> > the common mode input range in their spec's
> >
> > PC
> >
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