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RC filter at input of AD7767

Question asked by usaghi on Oct 14, 2013
Latest reply on Oct 22, 2013 by KarenNE



We do have following questions for RC filtering at input of AD7767.

Any of your answers, comments, or advises would be very much appreciated.



We are considering to add 0.1uF capacitance for eliminating noise at differential signal input between Vin+ and Vin- of AD7767,

into schematic of Figure 40 or 41 of Rev.C Datasheet shown in below.

Figure 40 Input filtering DS Rev.C .bmp

This means 52.47KHz cutoff for differential and 4.82MHz cutoff for common mode, subject to our calculation, so,

does this 0.1uF affect to any of sample-hold and result into degradation change of DC characteristics or conversion accuracy ?



We have old version datasheet Rev.0(local language translation) in our hand, and Figure 40 and 41 which I attached image.

(Sorry I could not insert it even though I could above.)

It has 3.3 ohm input resistance, and 10nF capacitance between Vin+ and Vin-, means 2.41MHz cutoff low-pass for diffrential signal.

However we are assuming it does not work for noise at common mode signal.

By looking at latest Rev.C datasheet as mentioned in above Question-1, it recommends 15 ohm and 2.2nF for common mode.

It seems input filtering had been changed, so regarding to previous question as well, could you please let us know is there any reason why ADI changed it, or a story behind ?



We assume the beginning of the first alias point of AD7767 has to be below 1.024MHz, at maximum.

However, as mentioned in above, latest datasheet indicates 4.82MHz cutoff input filtering, way high compare to above.

Please instruct reason why ?


Best Regards,