According to the NERNST Equation, the pH probe sensor produces an output of ±59mV/pH at room temperature, or 25°C, and ±71mV/pH at 85°C. Figure 7-8 shows how the pH probe can be modeled as a small, variable battery in series with a 10GΩ resistor. The probe impedance can vary significantly with temperature. As a result of the intrinsic characteristics of the pH probe, a near 0V output is produced for a neutral pH value of 7, but a ±30mV offset is common. This offset can be easily calibrated to 0V. The slope is given in manufacture data sheets, but a 2-point calibration can be done using a pH 4 or pH 10 buffer solution to confirm the probe is working properly.

Figure 7-8 pH Probe Simplified Model

A gain of 14V/V provides a wide output swing of approximately ±7V. To enable single-supply operation, a 7V reference voltage (V_REF) is created using the 15V supply voltage and a simple voltage divider. The output swing is shifted to 0V to 14V, and is conveniently proportional to the approximately ±1V/pH at 85°C. Figure 7-9 shows the resulting output voltage based on the theoretical pH sensor signal. In practice, pH probes show significant nonlinearity for very acidic and alkaline media; therefore, the measurement in Figure 7-9 is constrained to ±400mV. Temperature calibration of the pH sensor (not shown) is necessary for accurate results when wide temperature variation is expected.