A narrow range of frequencies around 1 kHz is rejected by the high Q filter shown in the figure, suggested by Texas Instruments. The operational amplifier used is of the type with field effect transistor at the input and the power supply used must be symmetrical. Note that the values of the components of the double T that determines the rejection frequency must maintain a very well-defined relationship of values. The formula for determining the rejection frequency as a function of component values is given along with the diagram. Other operational amplifiers with FET at the input can be used in the same configuration.
