Method and apparatus for the generation of long time constants using switched capacitors

摘要

A switched capacitor (12) providing an effective resistance having a relatively long time constant. The switched capacitor (12) includes a technique for switching an input switch (14) associated with the capacitor (12) between a first input potential and a second input potential where the second input potential is proportional to the first input potential. In one specific implementation, the capacitor is a primary capacitor (30) having a three position input switch (46) that is switchable between a ground contact (48), an intermediate contact (42) and the input potential, and a three position output switch (50) that is switchable between a first ground contact (56), a second ground contact (54) and an output potential. A secondary capacitor (32) includes a two position input switch (34) that is switchable between the input voltage and a ground contact (38), and an output switch (40) that is switchable between a ground contact (44) and the intermediate contact (42). The clocking frequency of the primary capacitor (30) enables the input switch (46) to be switched from the ground contact (48) to the intermediate contact (42) to the input voltage. When the input switch (46) of the primary capacitor (30) is connected to the intermediate contact (42), the secondary capacitor (32) is repeatedly switched between the input voltage and ground in order to provide the proportional input voltage. The number of times that the secondary capacitor (32) is switched when the primary capacitor (30) is connected to the intermediate contact (42) and the ratio of the capacitances of the capacitors (30, 32) determines the effective resistance.