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why Fluke use bandpass filter in their DMMs?
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emm386:
Hi all,

While I was skimming over the service manual of Fluke's 87-V and 189-II (figures attached), I realized that they have a bandpass-looking filter in their custom IC.
According to the Fluke 87-V service manual, it is referred as a low-pass filter, but its feedback structure makes it looking like an active bandpass filter (reference: https://cheever.domains.swarthmore.edu/Ref/FilterBkgrnd/Filters.html).
Can anyone please explain me the purpose of this filter and why this is configured as a bandpass filter ?

Plus, where would the 'FAMP_O' pin in the first attachment be connected?




Kleinstein:
The filter as 2nd order low pass filters. It is a configuration often found in DMMs, but not so much in the standard books.
In the first schematics C27+C36+R78 and the Filter op amp form a kind of "super (2nd order)" capacitor.
With a cross over at some 10 Hz (crude estimate) this filter is likely there to suppress mains hum (as part of the signal) before the gain stage. 
emm386:
Could you explain me how this forms a second order filter? Even in my LTSpice simulation this shows BP response...
Kleinstein:
The low pass ouput is not from the output of the OP-amp, but from the right of R2.
MegaVolt:
Perhaps these images will help you understand these filters.
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