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This circuit uses a AD645 op amp to form a pH probe amplifier. A typical pH probe requires a buffer amplifier to isolate its 1megohm to 1000 megohm source resistance from external circuitry. The low input current of the AD645 allows the voltage error produced by the bias current and electrode resistance to be minimal. The use of guarding, shielding, high insulation resistance standoffs, and other such standard methods used to minimize leakage are all needed to maintain the accuracy of this circuit. The slope of the pH probe transfer function, 50mV per pH unit at room temperature, has a +3300 ppm per degree C temperature coefficient. This circuit provides an output voltage equal to 1 volt per pH unit. Temperature compensation is provided by resistor Rt which is a special temperature compensation resistor, part number Q81, 1k ohm, 1%, +3500 ppm per degree C, available from Tel Labs Inc. Bypass capacitors ( 0.1uF ) should be used on each supply line.
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