Considerations for the design of switched-capacitor circuits using precise-gain operational amplifiers

Standard design of switched-capacitor (SC) networks assumes operation with infinite gain and infinite bandwidth opamps. However in the opamp design a tradeoff exists between the speed and the gain. As a consequence when a high sampling frequency is used, the needed large bandwidth limits the opamp gain to low values, therefore limiting the achievable accuracy. For this reason standard design is less feasible for high-sampling frequency. For these cases alternative design approaches are needed that address the opamp design tradeoff between speed-and-gain, allowing the frequency range of SC networks to be extended. A possible alternative is the precise opamp gain (FOG) design approach which has been already demonstrated with the realization of a 150 MHz SC filter reported elsewhere. The FOG design approach consists of designing high-frequency SC networks taking into account the precise gain value of the opamps as a parameter in the capacitor design. The standard opamp design tradeoff between speed-and-gain is changed into the FOG design tradeoff between speed-and gain precision which is more affordable in high-frequency opamps. In this brief, the FOG design approach is developed in detail and practical considerations regarding its possibility and limits are reported.