Patent ID: 8558592

Claim:
A circuit comprising: a first charge pump comprising a first pair of switches and a first pair of resistors, a junction of the resistors in the first pair of resistors forming a first node, wherein the first charge pump is operable to source a first current to the first node or to sink the first current from the first node by operation of the first pair of switches; a second charge pump comprising a second pair of switches and a second pair of resistors, a junction of the resistors in the second pair of resistors forming a second node, wherein the second charge pump is operable to source a second current to the second node or to sink the second current from the second node by operation of the second pair of switches; and a filter comprising an impedance and a first capacitor coupled in series between a third node and a fourth node of the filter, wherein a junction of the impedance and the capacitor forms a fifth node of the filter, and a second capacitor coupled between the third node and the fourth node, in parallel with the series combination of the impedance and the first capacitor, wherein the first node is coupled to the third node and the second node is coupled to the fifth node, and wherein one said resistor of said first pair of resistors and one said resistor of said second pair of resistors are selected to bear a ratio to one another such that, in a transfer function of the filter, a capacitance value of the second capacitor is multiplied by a predetermined amount that corresponds to said ratio; wherein the first capacitor determines a zero of the transfer function of the filter; wherein the second charge pump sinks the second current from the second node when the first charge pump sources the first current to the first node; wherein the second charge pump sources the second current to the second node when the first charge pump sinks the first current from the first node; and wherein the filter is an active filter and the impedance is provided by a first resistor, the second capacitor determining a pole of the transfer function of the active filter; and wherein the active filter further comprises a common-mode voltage generator to generate a common-mode voltage; and an operational amplifier (OPAMP), wherein an inverting terminal of the OPAMP is coupled to the third node, a non-inverting terminal of the OPAMP is coupled to the common-mode voltage, and an output terminal of the OPAMP is coupled to the fourth node.