Posted on Feb 6, 2014

3130 CMOS opamp gives required high input impedance for pH probe at low cost. Output of probe, ranging from positive generated DC voltage for low pH to 0 V for pH 7 and negative voltages for high pH values, is amplified in circuit that provides gain adjustment to correct for temperature of solution being measured. For analog reading, output of opa

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mp can be fed directly to center-scale milliammeter through 100K calibrating pot. For digital display giving reading of 7. 00 for 0-V output, pH output is converted to calibrated current for summing with stable offset current equal to 700 counts. This is fed to current-to-frequency converter driving suitable digital display. Standard pH buffer solutions are used for calibration. -D. Lancaster, "CMOS Cookbook, " Howard W. Sams, Indianapolis, IN, 1977, p 347-349.

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