why Fluke use bandpass filter in their DMMs?  - Page 1
Forum section: electronics / testgear

author: emm386
author_rank: Newbie
timestamp: October 04, 2023, 05:46:54 am
content: 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?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 04, 2023, 07:28:13 am
content: 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.

author: emm386
author_rank: Newbie
timestamp: October 04, 2023, 07:41:00 am
content: Could you explain me how this forms a second order filter? Even in my LTSpice simulation this shows BP response...

author: Kleinstein
author_rank: Super Contributor
timestamp: October 04, 2023, 08:20:57 am
content: The low pass ouput is not from the output of the OP-amp, but from the right of R2.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: October 04, 2023, 12:18:46 pm
content: Perhaps these images will help you understand these filters.

author: emm386
author_rank: Newbie
timestamp: October 04, 2023, 05:39:56 pm
content: MegaVolt and Kleinstein, thanks for the clarification and answering my question! this is very helpful

author: David Hess
author_rank: Super Contributor
timestamp: October 04, 2023, 09:11:55 pm
content: 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.

author: tszaboo
author_rank: Super Contributor
timestamp: October 04, 2023, 10:36:32 pm
content: Looking at the second picture, my question is rather:Why do they have so many lightbulbs in a multimeter?

author: David Hess
author_rank: Super Contributor
timestamp: October 05, 2023, 07:08:56 am
content: Quote from: tszaboo on October 04, 2023, 10:36:32 pmAs shown below, what light bulbs in multimeters might look like.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2023, 07:48:25 am
content: 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.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: October 05, 2023, 10:19:40 am
content: Quote from: David Hess on October 04, 2023, 09:11:55 pmDo you have an example circuit diagram?

author: tridac
author_rank: Regular Contributor
timestamp: October 05, 2023, 12:15:09 pm
content: 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...

author: MegaVolt
author_rank: Frequent Contributor
timestamp: October 05, 2023, 12:26:45 pm
content: Quote from: tridac on October 05, 2023, 12:15:09 pm

author: David Hess
author_rank: Super Contributor
timestamp: October 05, 2023, 12:40:06 pm
content: Quote from: MegaVolt on October 05, 2023, 10:19:40 amThe Tektronix DM501 and DM502 use it with a 318 type of operational amplifier which otherwise would contribute large DC errors.

author: tszaboo
author_rank: Super Contributor
timestamp: October 05, 2023, 03:40:47 pm
content: Quote from: Kleinstein on October 05, 2023, 07:48:25 amQuote from: David Hess on October 05, 2023, 07:08:56 amCome 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 05, 2023, 04:04:38 pm
content: internal switches, not lamps.check here for example:http://www.cyrustek.com.tw/en/wp-content/uploads/ES51997.pdf

author: David Hess
author_rank: Super Contributor
timestamp: October 05, 2023, 05:05:17 pm
content: Quote from: tszaboo on October 05, 2023, 03:40:47 pmAny light bulb is a switch, if driven hard enough, and the reverse.

author: TimFox
author_rank: Super Contributor
timestamp: October 05, 2023, 05:18:51 pm
content: Symbolism can be difficult.

author: tridac
author_rank: Regular Contributor
timestamp: October 06, 2023, 03:45:50 pm
content: 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...