Digital pH Meter Signal Conditioner with Pulse/Frequency Output

A signal conditioner for pH meter probe is required to have high input impedance. The signal conditioning of the pH meter probe is done by providing a buffer with gain, offset, and volatge to frequency converter. High input impedance requirement for pH probe while maintaining low cost constraint is given by 3130 CMOS opamp. Output of probe, is amplified by the op-amp. The configuration of the op-amp provides gain and offset adjustment to calibrate the drift caused by temperature variation of solution measured by the probe. This amplifier produce DC signal ranging from positive voltage for low pH to 0 V for pH 7 and negative voltages for high pH values. Output of opamp can be fed directly to center-scale milliammeter through 100K calibrating pot for analog reading. In digital display, 0V output representing pH 7, then pH output is converted to calibrated current for summing with stable offset current equal to 700 counts. This converter convert fed to current to frequency and very suitable for digital display. Here is the schematic diagram of the circuit: