why Fluke use bandpass filter in their DMMs?

[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.

[emm386]

MegaVolt and Kleinstein, thanks for the clarification and answering my question! this is very helpful

[David Hess]

The advantage of this filter configuration is that it is DC accurate; the DC offset voltage of the amplifier does not contribute to the output.  There is a similar configuration for a DC accurate precision rectifier which is AC coupled on its input and output.

[tszaboo]

Looking at the second picture, my question is rather:
Why do they have so many lightbulbs in a multimeter? 

[David Hess]

Why do they have so many lightbulbs in a multimeter?

As shown below, what light bulbs in multimeters might look like.

[Kleinstein]

In a few cases light bulbs are used as a low current fuse.
In the plan from the start the circle with x symbol is not for light bulbs but for CMOS switches.  So no light bulbs but chip internal CMOS switches for the signal.

P.s. : A light bulb is not a good fuse and may fail with an arc flash. It is not uncommon for a failing mains bulb to take out the breaker from that.

[MegaVolt]

There is a similar configuration for a DC accurate precision rectifier which is AC coupled on its input and output.

Do you have an example circuit diagram?

[tridac]

Been trying to find more info on this filter design. Asked in another thread and pointed towards a Burr Brown third gen op amp techniques book, since found on ABE books. Still interested in a more thorough treatment, so which book or source did you find the above 5th order example ?.

Thanks, still collecting data...

[MegaVolt]

Still interested in a more thorough treatment, so which book or source did you find the above 5th order example ?

[David Hess]

There is a similar configuration for a DC accurate precision rectifier which is AC coupled on its input and output.

Do you have an example circuit diagram?

The Tektronix DM501 and DM502 use it with a 318 type of operational amplifier which otherwise would contribute large DC errors.

[tszaboo]

In a few cases light bulbs are used as a low current fuse.
In the plan from the start the circle with x symbol is not for light bulbs but for CMOS switches.  So no light bulbs but chip internal CMOS switches for the signal.

P.s. : A light bulb is not a good fuse and may fail with an arc flash. It is not uncommon for a failing mains bulb to take out the breaker from that.

Why do they have so many lightbulbs in a multimeter?

As shown below, what light bulbs in multimeters might look like.

Come on, take a look at the picture, there are at least 20 light bulbs in that multimeter according to the schematic.
Or someone used that symbol as the selector switch's switching part as a lightbulb. If you look at it, 29-30-31-32 clearly sets a gain of an amplifier.

[joeqsmith]

internal switches, not lamps.

check here for example:
http://www.cyrustek.com.tw/en/wp-content/uploads/ES51997.pdf

[David Hess]

Come on, take a look at the picture, there are at least 20 light bulbs in that multimeter according to the schematic.
Or someone used that symbol as the selector switch's switching part as a lightbulb. If you look at it, 29-30-31-32 clearly sets a gain of an amplifier.

Any light bulb is a switch, if driven hard enough, and the reverse.

[TimFox]

Symbolism can be difficult.

[tridac]

There are references in that article, one (9), being Analog Filter Design by Val Valkenburg. Looked that up and lloks like it may have material on the filter topology of interest. Found a copy on ABE Books for less than $10, soon to be on it's way to me...