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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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