Tuning circuit for a filter

Tuning circuit for tuning a filter stage, wherein the filter stage comprises:

• (a) at least one RC element with an RC time constant, with the RC time constant being the product of the resistance of a resistor and the capacitance of a capacitor which is connected at a potential node to the resistor;
• and
• (b) an operational amplifier including a first signal input which is connected to the potential node and including a signal output which is fed back via the capacitor to the first signal input of the operational amplifier;
• wherein the tuning circuit comprises:
• (c) a comparator including a first input which is connected to the signal output of the operational amplifier and including a second input which is alternatively connectable to a first reference voltage source which generates a first reference voltage, or to a second reference voltage source which generates a second reference voltage;
• and
• (d) a controller which is connected to the comparator, the controller including a sequence controller,
• wherein the capacitor has its charge varied by means of switches which are controlled by the sequence controller, until the first reference voltage is applied to the first input of the comparator, and the capacitor then has its charge varied in the opposite direction by means of switches which are controlled by the sequence controller until the second reference voltage is applied to the first input of the comparator,
• wherein a digital counter contained in the sequence controller records a charge variation time for varying the charge of the capacitor;
• (e) wherein the controller switches a capacitor array as a function of the recorded charge variation time, and
• wherein the capacitor array is connected in parallel with the capacitor of the RC element in order to compensate for any discrepancy between the RC time constant of the RC element and a nominal value of the RC time constant.