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.


================================================================
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
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

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