ZHCSNI6 march 2023 OPA928
ADVANCE INFORMATION
In the Figure 8-3 example, the pH-probe sensor is assumed to produce an output of ±59 mV/pH at room temperature, or 25°C, and ±71 mV/pH at 85°C. The pH probe can be modeled as a small, variable battery in series with a 10‑GΩ resistor. As a result of the intrinsic characteristics of the pH probe, a near 0‑V output is produced for a neutral pH value of 7. A gain of 14 V/V provides a wide output swing of approximately ±7 V. To enable single supply operation, a 7-V reference voltage (VREF) is created using the 15-V supply voltage and a simple voltage divider. The output swing is shifted to 0 V to 14 V, and is conveniently proportional to the approximately 1 V/pH at 85°C. Temperature calibration of the pH sensor (not shown) is necessary for accurate results when wide temperature variation is expected.