This is quite [OT] but still related to the original PIC interfacing
problem.

The point here is that even using the pH electrode as an independent
element, just generating the diferential voltage to the op-amp, at least
one op-amp input (-) needs a DC voltage refence to keep all the signal
swing inside the op-amp input common voltage, so the input will not be
"clipped" by the input rail tolerances.  This is a "dc" input
polarization of the op-amp.

A simple 100k resistor to ground or Vcc,  or a voltage divider to
reference to middle vcc, can do the trick.  If you don't do this, the +
and - op-amp inputs will be dc floating and it can "travels" close to
ground or vcc, clipping the input.  As I said in my previous post, only
instrumentation amplifiers don't need this dc polarization, since the
reference is made internaly.

Feedback resistors double as dc polarization (unit or gain
configuration), if the output load goes to a valid input voltage swing.
The feedback to negative input will try to set the "float output ground"
to input.  As experience, install a 100 Ohms resistor at the op-amp
inputs, and a 10k directly from - input to the output (voltage
follower), another 10k from the output to VCC, measure the voltage at
the inputs. Now using a floating transformer apply 1Vpp senoidal signal
across the 100 Ohms resistor at input, scope the output. Move the 1k
load from VCC to ground, repeat the experience. Move the 1k load from
Ground to half VCC, repeat the experience.

Better and easy experience, install a 10k potentiometer extremes to VCC
and Ground, Center pin to the op-amp output via a 10k resistor. Rotate
Pot knob from one extreme to another while scoping op-amp output signal.
When you see some clipping it could not means only output rails
clipping, for voltage followers it is most likelly to be input rails
clipping.

Wagner

"Robert A. LaBudde" wrote:
>
> At 11:50 AM 10/19/99 -0400, Chris wrote:
> >>From what I can tell, you're suggesting to basically connect one end
> >of the electrode to 2.5V.  I'd then be running current through the
> >electrode and I'm not sure that's a good thing...
>
> 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.
>
> This is the same issue as running the Vss on the PIC at ground and Vdd at
> -5V. The chip doesn't care, so long as the voltage difference and polarity
> are right.
>
> The simplest approach among those suggested is to rig the LMC660 as a unity
> gain difference amplifier, followed by a 5x voltage gain stage. The input
> impedance is > 10^12 ohms, and only the difference voltage is amplified.
> The current flow is only in the pA range. The fact that the output of the
> difference amplifier carries a 2.5 V DC offset is irrelevant to the pH
> electrode and to the op-amp. The input +/- 0.5 V falls within the swing
> allowed with the LMC660, even with a 5 V supply.