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multi-channel multiple feedback bandpass filter phase consistence

Question asked by shafa on Sep 11, 2015
Latest reply on Sep 14, 2015 by harryh

We are building a multi-channel signal conditioning circuit, in which a Multiple Feedback Band Pass Filter is included for every channel. The frequency-phase curve shows a linear relationship near the central frequency zone, with a very large negative slope, this means if the resistors and capacitors we use are not of high accuracy, the actual phase delay for a sine wave input at central frequency will be unpredictable, and the phase consistence between these channels will in consequence not be guaranteed.

Figure 1 shows the magnitude and phase response for a 2-order MFBPF. If the tolerances for the capacitors and resistors are 5% and 1% respectively, the phase uncertainness at central frequency is about 40 degree. This phase uncertainness will come to 150 degree when the filter order is 8!  

Preliminary test shows our PCB board has a phase inconsistence of +/- 15 degree, which is not allowed for the final system. Can someone help and give us some advices to improve the channel consistence, while still keep the MFBPF design?


Figure 1 Magnitude and phase response for a 2-order BPF


Figure 2 Phase response for a 8-order BPF

Figure 3 Phase response for a 8-order BPF, but with high accuracy components