Abstract: A ground loop detector circuit and method for an instrument that is used with either a pH sensor or a conductivity sensor. In the instrument used with a pH sensor, an AC diagnostic signal is provided to the sensor. A high input impedance diagnostic signal monitor monitors the voltage at a node adjacent the output of the diagnostic signal source. The occurrence of a ground loop causes the voltage at the node to drop. The instrument used with the conductivity sensor, not only monitors current returning to the diagnostic circuitry from the sensor but also uses a high input impedance monitor to monitor the current leaving the diagnostic circuit to the sensor. The relationship between the current from the sensor and the current to the sensor can be used to determine if a ground loop has occurred as such a loop will cause the current from the sensor to be less than the current to the sensor.