Fluke 8840A/AF "VDC Ratio" - Page 1
Forum section: electronics / testgear

author: J-R
author_rank: Super Contributor
timestamp: January 14, 2023, 12:35:23 am
content: I noticed some Fluke "8840A/AF" models have a ratio function label by the front/rear switch and the rear terminals, but I couldn't find any further information on this feature. Has anyone seen any documents referencing this or used it?

author: AVGresponding
author_rank: Super Contributor
timestamp: January 14, 2023, 09:00:21 am
content: I've never seen this, do you have a pic?

author: J-R
author_rank: Super Contributor
timestamp: January 14, 2023, 09:21:57 am
content: Hmm, I have to give you high-post-count users some grief for not doing a simple image search!However, I've attached some images shamelessly ripped from the internet just so we don't have to deal with any dead links later...

author: HKJ
author_rank: Super Contributor
timestamp: January 14, 2023, 05:17:30 pm
content: I do not know the meter, but a guess could be:There is no switch for ratio and rear input do cannot measure current. Maybe the current switch will select ratio mode when rear input and current is selected?It is a guess and I would have expected some sort of marking at the current switches (except there is no space for it).

author: AVGresponding
author_rank: Super Contributor
timestamp: January 14, 2023, 05:44:50 pm
content: Quote from: J-R on January 14, 2023, 09:21:57 amI did, but I misunderstood what you wrote; I was looking for a separate button.It's a DC/DC voltage ratio measurement function... It'll give you a ratiometric reading between a reference voltage (0-20VDC) and an unknown voltage (0-1000VDC). Maybe normal VDC readings are possible at the rear using only the "input"... inputs, and leaving the "reference" inputs floating.Manuals specific to the A/AF seem difficult to find.

author: bdunham7
author_rank: Super Contributor
timestamp: January 14, 2023, 07:04:59 pm
content: I don't have one here at the moment, so I can't demonstrate. However, it is pretty simple and similar to the X-REF feature found on other Fluke DMMs.The meter works from the front panel in a completely normal fashion, exactly like the regular 8840A.If you push in the F/R switch, it displays the ratio between the X-REF and the other rear input. The X-REF is limited to +/- 20V, the other to 1000VDC. The X-REF is measured just like any other voltage and the comparison is done by the CPU--this is not a direct substition of the internal ADC reference with X-REF or anything like that. There may be a lower limit for X-REF or a maxium ratio before you get an error, but I don't know what those limits are. Apparently this specific feature was added to meet a USAF requirement, although I have no details regarding that. Indeed, the manual does appear to be MIA--I thought I had one but can't find it.

author: J-R
author_rank: Super Contributor
timestamp: January 15, 2023, 04:13:36 am
content: OK, thanks for the comments. I have a regular 8840 and 8842, so was curious if this was a simple modification that could be made if you wanted that feature or if it's quite a bit more involved.

author: J-R
author_rank: Super Contributor
timestamp: January 15, 2023, 04:19:30 am
content: What got me going on this was I saw a regular 8840A with the ratio feature, but now I think either the display cover was replaced or perhaps there are AF models without the AF printing.

author: bdunham7
author_rank: Super Contributor
timestamp: January 15, 2023, 03:43:10 pm
content: Quote from: J-R on January 15, 2023, 04:13:36 amAFAIK they all use the same main board, so it would be possible. You'd need different firmware and there is a divider resistor Z201 that is added, presumably to divide X-REF down to +/-2V. There is an empty slot for Z201 in the right front corner of the board in all versions that I can remember seeing. If you really wanted DC ratio, I think you could do it starting with an 8840A, but there is no firmware available that would work with the 8842A so you'd have to deal with that. Does all that count as "quite a bit more involved"?You would be giving up the (very handy) ability to connect a 4W ohms kelvin-clip harness to the back inputs.