Fluke 87 high input impedance - Page 1
Forum section: electronics / testgear

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 06, 2025, 04:09:34 pm
content: I had the original Fluke 87 since 1988 until 2004. I don't remember that it has the high input impedance mode. I just read the manual and found that it has a 4Gigaohm input impedance mode. The newer 87 like 87v doesn't have this. Anyone remember this feature?

author: Martin72
author_rank: Super Contributor
timestamp: May 06, 2025, 04:24:33 pm
content: Oh,We still have an '87 model from 1990 at work, I'll have to try that out soon.I've never heard of that before.But I'm not surprised that this feature is no longer included in newer models, as the usual application does not require such high impedance, quite the contrary.

author: bdunham7
author_rank: Super Contributor
timestamp: May 06, 2025, 04:33:53 pm
content: Quote from: BeBuLamar on May 06, 2025, 04:09:34 pmThere were (and are still) some handheld meters that have high-impedance in their lowest range(s) but I don't recall seeing one like that where it is selectable as a power-on option. The 8060A has a selectable Hi-Z setting for the 200mVDC and 2VDC ranges. The 867B has selectable Hi-Z in the 300mVDC and 3VDC ranges. I think some current Metrix models also have selectable Hi-Z. I suspect that Fluke eliminated that feature as they started to conform to the IEC CAT ratings.

author: floobydust
author_rank: Super Contributor
timestamp: May 06, 2025, 04:37:49 pm
content: For the 87's I only know their input impedance as 10MEGSome Beckman/Wavetek handhelds 22MEG.OP are you thinking of the 34401A? It's 10MEG or special 10GIG for 100mV, 1V, 10VDC ranges.

author: bdunham7
author_rank: Super Contributor
timestamp: May 06, 2025, 04:57:07 pm
content: Quote from: floobydust on May 06, 2025, 04:37:49 pmI've never noticed, but there it is:

author: floobydust
author_rank: Super Contributor
timestamp: May 06, 2025, 05:09:19 pm
content: I couldn't find it in various 87 I have manuals back to 1989. Which model are we thinking?On the 400mV AC range, pressing Hz enters freq. counter mode. I think it's more likely the gate input impedance that is that high, a bum spec in the manual?Somebody would have to try measure it I guess to confirm.

author: 2N3055
author_rank: Super Contributor
timestamp: May 06, 2025, 05:20:51 pm
content: Metrix MTX 3293 has High-Z mode (1GΩ) on 100mV and 1V range

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 06, 2025, 05:28:57 pm
content: Quote from: bdunham7 on May 06, 2025, 04:57:07 pmI remember it now that I did know about it but never used it because it's only good for the 400mV DC range.

author: bdunham7
author_rank: Super Contributor
timestamp: May 06, 2025, 05:40:57 pm
content: Quote from: floobydust on May 06, 2025, 05:09:19 pmhttps://assets.fluke.com/manuals/87______umeng0800.pdf(page 14)It is a power-on option. Hold the HZ button down as you turn the meter from off to on, then select the mVDC range. It shouldn't be hard to confirm it is Hi-Z, I think the open-circuit readings will float up like any other Hi-Z meter.

author: Per Hansson
author_rank: Unknown
timestamp: May 06, 2025, 06:17:29 pm
content: Interestingly the Fluke 28-II has this feature too, I tested and it does really change the input impedance.As a side-note the original UT61E has a input impedance according to the manual of 3000MΩ in mV range.And it matches well for a couple depleted CR2032 batteries, if I measure the same using 10MΩ input impedance the voltage drops rapidly on the batteries...Quote

author: newbrain
author_rank: Super Contributor
timestamp: May 06, 2025, 06:37:42 pm
content: Quote from: BeBuLamar, heavily snipped on May 06, 2025, 04:09:34 pmWhat?Of course it does!Same as the original, hold (Hz %) button when powering on.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 06, 2025, 06:41:30 pm
content: Quote from: newbrain on May 06, 2025, 06:37:42 pmMy bad yes it does.