LMC660 is a nice amplifier, low cost, low drift, low consume and more,
works great in single +5V supply, one of the type that catch the eye for
price/performance, but with the high impedance input CMOS there is a
catch; is the crazy oscillation if configured as a follower (output
connected to the negative input). The manufacturer recommend to install
a RC network (see documentation) to avoid oscillations in this
configuration.  By other side, if you need a cheap oscillator, just
choose the right RC... :)

Your doubt about negative input, or inverting buffer with a positive
input, is the most common and old problem in instrumentation. This is so
often that they invented the "instrumentation amplifier".  You just need
a reference lower than the signal itself.  Those amplifiers accept
several types of transducers and sensors, mostly bridge configurations,
and they amplify the difference between the + and - inputs.  The same
for any other amplifier? yes, but the negative feedback is NOT done to
the - input, so it doesn't interfere with the input itself.  Those
amplifiers (as any other) need the inputs inside the common mode voltage
(Power + and -), and they are great to reject noise at both inputs
(common mode reject), as 60Hz for example.  Instrumentation amplifiers
can be build with 3 regular op-amp (LMC660 for example), but I usually
do it with only 2 with the same effect, so one LMC660 can produce 2
instrumentation amplifiers.  The good point is that in your case, the
inverting output just need a fixed reference voltage at the + input to
work as adjustment to the output dc component setup (what voltage will
be at the output when the input is zero).  A good inst-amplify cost more
than $10 while the LMC660 is around $2.

If you need some drawing examples, just email me.

Wagner

"Robert A. LaBudde" wrote:
> At 11:27 PM 10/18/99 -0400, Sean wrote:
> >How can a non-inverting follower work without at least a negative supply? I
> >interpreted "negative reference" to mean "negative supply" in this case.
> >
> >If we could use an inverting buffer, THEN it wouldn't need a negative
> >supply. However, good luck finding a high enough Rin,
>
> As far as I know, you never need to have bipolar supplies. You just need a
> Vss and a Vdd and a common point in between, typically at the midpoint.
>
> The common voltage becomes the signal ground. The full Vcc becomes the
> 'plus' voltage. The power supply ground becomes the 'minus' voltage.
>
> This works for all op-amps, although rail-rail types are best suited to low
> voltage supplies, such as the 5 V needed in the example.
>
> To connect the pH electrode to a non-inverting voltage follower with a
> LMC660, connect one side to the non-inverting input and the other side to
> the common point (@ 1/2 VCC). Feedback the output to the non-inverting
> input. Voila!