Offset correction for sensor with temperature dependent sensitivity

A sensing device which can be trimmed to automatically compensate for output signal offset, comprising:

• an electrically energizable sensing element including output terminal means, and having a response to a first condition, said sensing element being operable to produce an analog voltage at said output terminal means representative of the first condition, the response of said sensing element being affected by a second condition, offset in said analog voltage being variable by varying current through said output terminal means;
• a tracking circuit connected for enerigization in parallel with said sensing element and including a fixed resistor, said sensing element producing an electric current whose magnitude tracks the response of said sensing element with respect to the second condition;
• and
• a current control circuit including adjustable resistor means whose resistance is substantially independent of the second condition, said adjustable resistor means being operable to proportion the current produced by said tracking circuit, said current control circuit being connected to the output terminal means of said sensing element and operable to control the current therethrough in response to an adjustable fraction of the current produced by said tracking circuit to vary the voltage offset at the output terminal means.

A differential current source for producing first and second precise currents whose sum is determined by a control current, comprising:

• a voltage supply conductor;
• a reference voltage conductor;
• first and second output terminals for supplying first and second currents;
• a control terminal for receiving a current whose magnitude is indicative of the sum of the currents supplied at said first and second output terminals;
• first, second and third transistors having base electrodes connected together and emitter electrodes connected to said reference voltage conductor, said first transistor having a collector electrode connected to said first output terminal and said second transistor having a collector electrode connected to said control terminal through a first resistor;
• fourth, fifth and sixth transistors having base electrodes connected together and emitter electrodes connected to said reference voltage conductor, said fourth transistor having a collector electrode connected to said second output terminal and said fifth transistor having a collector electrode connected to said control terminal through a second resistor;
• a seventh transistor having an emitter electrode connected to the base electrodes of said first, second and third transistors and to the collector electrode of said sixth transistor, a base electrode connected to the collector electrode of said second transistor and a collector electrode connected to said voltage source conductor;
• and
• an eighth transistor having an emitter electrode connected to the base electrodes of said fourth, fifth and sixth transistors and to the collector electrode of said third transistor, a base electrode connected to the collector electrode of said fifth transistor and a collector electrode connected to said voltage supply conductor.

A method for providing temperature independent output voltage offset adjustment for a Hall effect element having first and second supply terminals and first and second output terminals, comprising the steps of:

• providing a resistor having substantially the same temperature response as the Hall effect element;
• supplying the same voltage across the first and second supply terminals of the Hall effect element and across means including the resistor to produce a current through the resistor which tracks current through the Hall effect element with temperature;
• dividing the current through the resistor into first and second complementary current portions;
• controlling current flow through first and second output terminals of the Hall effect element in response to the first and second current portions respectively;
• and
• adjusting the ratio of the first and second current portions to achieve a null voltage between the first and second output terminals when no magnetic field is applied to the Hall effect element.