Differential mode analog active filter?
- From: Vladimir Vassilevsky <antispam_bogus@xxxxxxxxxxx>
- Date: Sat, 30 Jun 2007 22:58:27 GMT
Hello All,
There is a high precision ADC with the differential input (SNR ~ CMRR ~ 100dB). The incoming analog signal is differential also. However there should be the antialiasing filter in front of the ADC. The filter is Bessel 4th order. This is the high performance design, and the cost of the components is not a big issue.
What would be the best way to do this circuit without sacrificing the SNR and CMRR performance?
If we make the two identical analog filters for /+/ and /-/ paths, then the CMRR will suffer because of the limited tolerance of the components. I would expect problems mainly due to the accuracy of the capacitors.
Making a filter with fully differential opamps does not seem to be very different from the two separate filters. There is the same limitation because of the tolerances.
It is possible to collapse the input differential signal into the single ended using an inamp, then filter it and convert it back to the differential in front of the ADC. The CMRR will be limited to that of the inamp, and the SNR is in danger because of the conversion from the differential to ground referenced and back.
I tried to design a schematic similar to the conventional filter but with the floating ground. A simulation shows problems with the CMRR and the frequency response; the bootstrapping of the power rails of the opamps towards the floating ground helps somewhat, however it rather complex and doesn't look as a very reliable solution.
What topology should be the good approach to the problem? Can you suggest a book, an appnote or any other information?
VLV
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