20th-Order Measurement Filter

This circuit is based on the configuration of a fifth-order Butterworth filter using only one opamp (see p 116 of the 1995 Summer Circuits issue). Here we achieve a 20th-order filter by connecting four fifthorder sections in series. The first three sections are tapped off at TP1, TP2 and TP3. As you will see, the transfer characteristic is not a pure 20thorder Butterworth, but it does have the steepness of such a filter. The desired bandwidth for the whole filter is achieved by adjusting the turnover point of each section to a higher value.