Ground loop detector circuit and method

In an instrument which is used in combination with a sensor in electrical contact with a process for measuring a variable of said process and providing a DC signal having a level representative of said process variable, a method for detecting the occurrence of a ground loop comprising the steps of:

• a) providing an AC diagnostic signal with a known frequency to said sensor;
• b) measuring by a high input impedance monitor the voltage at a node that is between said source and said sensor, a predetermined impedance at said known frequency connected between said source and said node;
• and
• c) providing a return path from said sensor to said source by a low impedance input circuit that acts as a virtual ground to said DC signal level and said AC diagnostic signal thereon;
• whereby the occurrence of a ground loop causes the voltage at said node to decrease by an amount that is inversely related to the impedance of said ground loop.

In an instrument which is used in combination with a sensor that is in electrical contact with a process to measure a variable of said process, a method for detecting the occurrence of a ground loop comprising the steps of:

• a) providing an AC diagnostic signal current with a known frequency to said sensor;
• b) measuring said provided AC diagnostic signal current;
• c) connecting a predetermined impedance at said known frequency between said source and said provided current measuring means;
• and
• d) measuring the amount of said provided AC diagnostic signal current which returns to said source from said sensor;
• whereby the occurrence of a ground loop causes the voltage at said means for measuring the amount of said provided AC diagnostic signal current which returns to said source from said sensor to decrease by an amount that is inversely related to the impedance of said ground loop.

In combination:

• a) a sensor that is in electrical contact with a process to measure a variable of said process and provide a DC signal having a level representative thereof;
• and
• b) an instrument including a circuit for detecting the occurrence of a ground loop, said ground loop occurrence detecting circuit comprising:
• i) a source for providing an AC diagnostic signal with a known frequency to said sensor;
• ii) a high input impedance monitor connected to a node between said source and said sensor;
• iii) a predetermined impedance at said known frequency connected between said source and said node;
• and
• iv) a low impedance input circuit that acts as a virtual ground to said DC signal level and said AC diagnostic signal, said low impedance input circuit connected to said sensor to thereby provide a return path from said sensor to said source;
• whereby the occurrence of a ground loop causes the voltage at said node to decrease by an amount that is inversely related to the impedance of said ground loop.

In combination:

• a) a sensor that is in electrical contact with a process to measure a variable of said process;
• and
• b) an instrument including a circuit for detecting the occurrence of a ground loop, said ground loop occurrence detecting circuit comprising:
• i) a source for providing an AC diagnostic signal current with a known frequency to said sensor;
• ii) means connected to said source for measuring said provided AC diagnostic signal current;
• iii) a predetermined impedance at said known frequency connected between said source and said provided current measuring means;
• and
• iv) means for measuring the amount of said provided AC diagnostic signal current which returns to said source from said sensor;
• whereby the occurrence of a ground loop causes the voltage at said means for measuring the amount of said provided AC diagnostic signal current which returns to said source from said sensor to decrease by an amount that is inversely related to the impedance of said ground loop.