HQ Notch Filter Without Close-Tolerance components

Posted on Feb 6, 2014

A notch for a narrow frequency band of a few per cent or even less normally requires close-tolerance components. At least, that`s what we thought until we came across a special opamp IC from Maxim. In filters with steep slopes, the component tolerances will interact in the complex frequency response. This effect rules out the use of standard toler

HQ Notch Filter Without Close-Tolerance components
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ance components if any useful result is to be achieved. The circuit shown here relocates the issue of the value-sensitive resistors that determine the filter response from visible` resistors to ready available integrated circuits which also make the PCB layout for thelter much simpler. The operational amplifiers we`ve in mind contain laser-trimmed resistors that maintain their nominal value within 1 ° or less. For the same accuracy, the effort that goes into matching individual precision resistors would be far more costly and time consuming. The desired notch (rejection) frequency is easily calculated for both R-C sections shown in Figure 1. The circuit separates the amplitude and frequency domains using two frequency-determining R-C networks and two level-determining feedback networks of summing amplifier IC2, which suppresses the frequency component to be eliminated from the input signal by simple phase shifting. IC1 contains two operational amplifiers complete with a feedback network. The MAX4075 is available in no fewer than 54 different gain specifications ranging from 0. 25 V/V to 100 V/V, or +1. 25 V/V to 101 V/V when non-inverting. The suffix AD indicates that we are employing the inverting version here (G = 1). These ICs operate as all-pass filters producing a phase shift of exactly 180 degrees at the roll-off frequency f0. The integrated amplifier resistors can be trusted to introduce a gain variation of less...

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