Switched capacitor circuits are used in a variety of applications ranging from audio A/D converters to analog filter circuits. At its most basic level, switched capacitor circuits perform analog signal processing in the charge domain by sampling charge on capacitors. By using a combination of feedback amplifiers, switching devices and ratioed capacitors, various sampled analog transfer functions may be accurately represented even in the presence of a high amount of component value variation.
As the geometry of semiconductor processes has been shrinking, and as the demand for low power devices has been increasing, there has been a corresponding decrease in supply voltages for the purpose of preventing device breakdown and damage in smaller geometry devices and to reduce power consumption. In order to maintain headroom in switched capacitor circuits, various supply and clock boosting techniques have been employed to operate switching transistors. For example, given a 1.2 V supply voltage and a 0.7 V transistor threshold, an NMOS switching transistor would only have a compliance range of about 0.5 V if the full supply voltage of 1.2V were used to turn-on the transistor. If, on the other hand, a boosted supply or boosted gate signal is used, for example, to produce a 2V gate drive, the switching transistor may operate over a compliance range encompasses the entire 1.2 V power supply range.
Even with lower power supply voltages, however, there is still a need for switched capacitor circuits that are operable in the presence of input signals having voltages that may exceed the provided supply voltage, for example, in industrial and in automotive applications.