Budgeting Input Current Error for a pH Electrode Interface

A pH electrode presents a high source resistance, so a small input current can produce a meaningful voltage error. A low offset amplifier is not automatically a suitable buffer if its bias current or the surrounding circuit’s leakage dominates the result. Review the complete input path before calibrating the finished instrument.

Estimate the voltage error scale

Use the electrode manufacturer’s resistance information over the relevant temperature and condition range. As an illustrative calculation, 10 pA flowing through 100 megohms produces 1 mV. This establishes the sensitivity to input current; it does not define a universal electrode resistance or pH accuracy.

The Analog Devices CN0326 pH interface note uses a low input bias current buffer for the high impedance electrode and discusses the effect of source resistance. Check maximum and temperature dependent input current data for the actual amplifier rather than relying only on a room temperature typical value.

Count leakage outside the amplifier

Include protection devices, connector insulation, contamination, multiplexers, and any diagnostic network connected to the input. Trace where each leakage current returns. A protection component can satisfy a voltage rating while adding unacceptable leakage for this measurement.

Review the input node under expected humidity and temperature conditions. Keep the test fixture as clean and well insulated as the intended board; otherwise fixture leakage may be mistaken for circuit behavior. Record cleaning and handling conditions when comparing units.

Test the electronics with a source model

Apply a known voltage through representative high resistance values and compare the measured result with a low resistance source. Allow adequate settling after each connection or voltage change. A quick reading can mix steady leakage error with charging of input and cable capacitance.

Repeat with input polarity and operating temperature where practical. Check whether the error follows source resistance, which helps distinguish current related effects from a nearly constant voltage offset. Avoid adjusting calibration coefficients until the mechanism is understood.

Keep electrode behavior separate

Electrode condition, temperature compensation, reference junction behavior, and calibration solutions remain part of the complete pH measurement. An electrical simulator verifies the front end but cannot establish that an actual electrode is correctly maintained or calibrated.

Save the assumed source resistance range, leakage budget, simulator setup, settling interval, and measured residual error. Then perform the required electrode calibration using the sensor’s procedure. Separating these two stages makes it possible to identify whether a later drift belongs to the PCB electronics, the connection, or the electrochemical sensing element.