At 07:50 PM 10/19/99 -0400, Chris wrote:
>> One point of voltage reference doesn't make a circuit. It is irrelevant
>> what the common point signal voltage is, since the pH electrode is not at
>> earth ground, and the common signal voltage is not connected to earth
ground.
>
>The problem is, the solution conducts.  (ie. saltwater)  There will
>also be other probes that will require different offsets.  I'll have
>to stick with the instrumentation amplifier configuration for everything
>it appears.
>
>Now I remember why the idea of running current through it bothered me.

I must be missing something in this problem. I don't understand the
connection to the conductivity of the solution.

Since the 2.5 V is a common voltage across the electrode, it wouldn't
matter if it was a copper wire instead: no current will flow from a
constant potential on both terminals.

The only current as I can see it which will be drawn from the pH electrode
is the bias current from the op-amp, which is a fraction of a picoamp, so
should be no problem.

The only issue that's been raised so far is in connecting the LMC660 as a
unity gain non-inverter, since the inputs are not rail-rail. This is not a
problem with the pH electrode, since a +/- 0.5 V swing lies within the
allowed range even at 5 V.

I use the LMC660 with battery supplies all the time with either a voltage
divider midpoint reference or a buffered version (since I've got 4 op-amps
on the chip). I've never had a problem of the type discussed.

I'd be interested in hearing a good reason for a bipolar supply, other than
historical ones.

I'm not an EE, so perhaps I'm either not understanding the problem or some
trade-off involved. I am, however, a chemist, so I have some understanding
of  pH electrodes and electrochemical cells. Anything under a nanoamp
shouldn't be a problem.

PS. A single-supply design white paper can be found at:

http://www-s.ti.com/sc/psheets/sloa030/sloa030.pdf



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Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral@lcfltd.com
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