At 06:50 AM 6/11/2004 -0700, you wrote:
>Regarding "straining the op amp" in the non-inverting mode, I believe the
>writer was refering to the fact that in non-inverting mode, there is a
>large common mode voltage on the input in non-inverting mode, while the
>common mode voltage in inverting mode is zero.

Not necessarily zero (depending on the configuration), but a fixed value
that does not vary with the input voltage. Of course you can run into
problems with the classic non-inverting follower (gain = 1) with common
mode *input* range, even if the *output* is rail-to-rail.

One subtle issue that is easy to miss is that the offset voltage of
rail-to-rail I/O op-amps can undergo a shift as the input stage changes
from one type to the other (with common mode voltage range). This can
result in nonlinearity in the transfer function if the common mode
voltage moves through that region (which also could be expected to
vary somewhat from unit to unit). The effect is rather large (hundreds
of microvolts) in some op-amps.

I use non-inverting in many very precise measuring instruments, but
you do have to be aware of all the gotchas with the parts you are using.
For example, if the input voltage is only 100mV full scale, it's "almost"
constant common mode input voltage.

Another issue is that the characteristics change substantially as you
approach saturation in some kinds of op-amps, so you may see some ill
effects at 100mV output voltage when full saturation is more like 40mV.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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