The circuit shows how to use the very high-input impedance of a MAX406 CMOS op amp to buffer the extremely high-output impedance of a pH probe. The advantage gained is the use of a standard low-cost cable instead of a high-cost Teflon dielectric coax cable. Battery drain is so low that the circuit can be continuously powered for thousands of hours on a single lithium cell. Use of a MAX130A integrating ADC is shown for driving an LCD display.
The extremely high source impedance of a pH probe (1012Ω) often mandates the use of low-loss Teflon cable between the probe and its meter electronics. Such cable costs several dollars per foot. As an alternative, you can buffer the probe with a MAX406 low-power op amp that allows the use of ordinary, less-expensive coaxial cable instead. The resulting system (Figure 1) includes a general-purpose pH electrode, a buffer circuit, and a simple LCD-meter circuit based on a 3½-digit, integrating-type A/D converter (IC2).
If desired, you can install the op amp and its power supply (a small lithium battery) within the probe housing. The entire probe-interface circuit consumes less than 1.5µA, and operates for thousands of hours on the DL1620B lithium cell shown.
The resistive divider R1/R2, whose output is midway between the 3V battery's terminals, provides a reference potential for the pH probe. This potential, connected via the coax shield to pin 32 of IC2, also establishes a common-mode reference for the A/D converter. (Pin 32 is generally 3V below the converter's V+ level at pin 1.) Potentiometer R3 introduces an adjustable 700mV offset. By shifting the probe's ±700mV output range to one of 0 to 1400mV, this offset provides an output compatible with the intended display range of 0 to 14pH.