Help understanding schematics - Page 1
Forum section: electronics / general

author: eduaqa
author_rank: Contributor
timestamp: February 06, 2025, 12:11:13 pm
content: I'm trying to fix a fluke 867b, with the following problem:The functions on dial position on the left works, let say from the last position Amp to VDC, then don't works from VAC to LOGIC, so I'm thinking the problem should be the voltage divider on the switchI downloaded the Service Manual which is available on fluke site itselfWhile looking on the schematics (last pages), on the switch I found where the switch is detailed, but there are some "symbol" that I don't the meaning (see the picture)What are those triangles? I thought would mean Ground, by they have the letter "D" and the number "3" inside ... I thing just a triangle means Ground, but what they means with letter and number inside? What the difference between the one with "D" and the one with "3"?

author: pqass
author_rank: Super Contributor
timestamp: February 06, 2025, 12:52:13 pm
content: Possibly:"D" for digital ground; where all noisier switching component grounds meet,"3" for a particular analog ground where all "3"s meet.It's very common to separate the digital from analog grounds so any digital noise isn't amplified by the sensitive analog parts.Eventually, all grounds will meet at one point. Seethis.

author: wn1fju
author_rank: Frequent Contributor
timestamp: February 06, 2025, 12:52:50 pm
content: On the first page of the schematics section of the service manual, you will see a list of the six grounds in the system. For instance, the D ground is the digital supply ground and the 3 ground is one of the analog grounds. The triangles on the schematics are the various grounds with the number/letter being an abbreviation for which of the six.It is very common to have multiple grounds especially when mixing digital and floating analog circuits. If you look through the schematics very carefully, you might find one or more of the grounds are tied together (or you could use an ohmmeter and figure it out yourself).However, when grounds are not all tied together and you are debugging your equipment, you have to be careful to use the correct ground when making voltage measurements. I've been burned by this before!

author: tom66
author_rank: Super Contributor
timestamp: February 06, 2025, 12:55:54 pm
content: Just a note that there is a lot of mythology around "digital" and "analog" ground and most EMC engineers will tell you to keep a single common ground for everything, and indeed good design of any analog system you should not need a separated ground just careful placement of digital and analog components. Splitting ground planes can introduce more noise than having a common ground as any split will risk turning both halves of the ground plane into a radiating dipole.One good case for having a separate ground is if the meter's COM terminal is not referenced to battery ground. In some cases this will be connected to something like a split supply, e.g. +3V from the battery ground, which will allow the ADC and opamps to all use a simpler power supply, with the battery ground used as their 'negative' supply. So the '3' could indicate this virtual ground (just a guess on my part, but I've seen it used in non-Fluke meters).