The OPA928 behaves very close to an ideal op amp in regards to the input current. The near-zero input bias current performance enables applications with extremely high impedance signal sources. For example, pH probes can have an output impedance of up to 10GΩ. Most op amps are inadequate to use with this kind of sensor impedance. For example, a CMOS op amp with 1pA of input bias current loads the sensor and causes a large, and unacceptable, 10mV error at the input at room temperature. This error can increase exponentially across the industrial temperature range. In Figure 7-7, the OPA928 is used as a high-impedance electrometer to amplify the small signal from the pH probe sensor. The high input impedance and ultra-low bias current into the positive input pin of the OPA928 does not load the sensor and minimizes the input bias current error.

Figure 7-7 High Impedance pH Probe Amplifier Circuit