Sorry, Robert, you are right. I was thinking of cases where the input is
already GND referenced, THEN you do need the bipolar supplies,since you
can't float it. However, in this case you are right.

As for the concern over the current thru the electrode, there shouldn't
be any current. The input impedance of the amp is still very high,so
raising the electrode to 2.5v doesn't make any difference.

Sean



On Tue, 19 Oct 1999, Robert A. LaBudde wrote:
> 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!
>
> What's the problem?
>
> ================================================================
> Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral@lcfltd.com
> Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
> 824 Timberlake Drive                            Tel: 757-467-0954
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>
> "Vere scire est per causae scire"
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>