Ultra low cut-off frequency filter

A filter circuit having a selectively set cut-off frequency comprising:

• a control circuit having a first input terminal, a second input terminal, and an output terminal, the first input terminal configured to receive an input signal from an input source, the second input terminal configured to receive a feedback signal, the control circuit operable to generate a control signal at the output terminal;
• a controllable resistor coupled to the control circuit and having a first terminal, a second terminal, and a third terminal, the first terminal configured to receive a reference signal from a reference source, the second terminal configured to receive the control signal from the control circuit, to generate the feedback signal, a resistance of the controllable resistor being proportional to a voltage difference of the reference signal and the input signal wherein the resistance of the controllable resistor is inversely proportional to a current at output of the controllable resistor;
• a feedback path coupled to the control circuit and the controllable resistor to couple the feedback signal from the controllable resistor to the control circuit, the control circuit controlling the controllable resistor to adaptively minimize leakage voltage drop thereacross by modulating the resistance thereof, thereby preventing noise from at least one of the input signal and the reference signal from appearing at an output of the filter circuit, and preventing voltage error at the output of the filter circuit;
• and
• a capacitor device having a first terminal and a second terminal, the first terminal of the capacitor coupled to the third terminal of the controllable resistor and the second terminal of the capacitor coupled to one of an electrical ground and a voltage source, to filter one or more frequencies.