Low drift tuning circuit

A low drift circuit powered by first and second power supply buses and operated in conjunction with a feedback network, the circuit comprising:

• an operational amplifier having positive and negative input terminals, an output terminal, and first and second power terminals connected respectively to said first and second power buses, said operational amplifier having a common mode input voltage, the output terminal of said operational amplifier connected to an input terminal of said feedback network,
• a first voltage divider circuit connected between an output terminal of said feedback network and the negative input terminal of said operational amplifier for supplying an input signal thereto;
• a reference node having a fixed voltage applied thereto, said fixed voltage slightly offset from the voltage on said second power bus to minimize said common mode input voltage, said reference node connected to the positive input terminal of said operational amplifier;
• and
• a capacitor connected between the output terminal of said operational amplifier and the negative input terminal thereof for integrating an output signal from said operational amplifier.

A method for reducing the drift of a tuning circuit powered by first, second and third power buses where the second bus is at a voltage between the voltages of the first and third buses, comprising the steps of:

• connecting the power terminals of an operational amplifier to said first and second power buses;
• connecting a negative, inverting input of said operational amplifier to said second power bus to set the common mode input voltage of said operational amplifier at essentially zero volts referenced to said second power bus, said common mode input voltage being proportional to the input leakage current of said operational amplifier;
• transmitting the output signal from said operational amplifier through a level shifting and inverting amplifier;
• connecting the output of said inverting amplifier to a feedback network;
• and
• connecting the output of said feedback network to the positive, noninverting input of said operational amplifier.

A method for reducing the drift of a tuning circuit powered by first and second power buses which operates with a feedback network, comprising the steps of:

• connecting the power terminals of an operational amplifier to said first and second power buses, said operational amplifier having an input leakage current proportional to a common mode input voltage;
• coupling the output signal of an operational amplifier to an input of said feedback network;
• coupling the output signal of the feedback network to one of the input terminals of said operational amplifier which has a positive, non-inverting input terminal and a negative, inverting input terminal;
• and
• connecting the other of the input terminals to a reference voltage node which has a fixed voltage slightly offset from the voltage of said second power bus such that said common mode input voltage at the input terminals of said operational amplifier is approximately zero.