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 a LCD display.