EEVblog Electronics Community Forum

Products => Test Equipment => Topic started by: BoneHead on February 25, 2016, 01:27:00 am

Title: Another Fluke 8840 question
Post by: BoneHead on February 25, 2016, 01:27:00 am
Browsed / searched for this but didn't find anything.

Have a couple of 884os (plain A & an A/AF) and both respond in the same way described here.

When i make a DC voltage measurement, the measured DC voltage is correct; however when the probes are removed from the DC voltage source the readouts (display) don't return to zero or auto-range down they stay up around the max measured DC voltage.  i.e. I measure 14.80Vdc form a power supply, display shows 14.7970 Vdc, removed probes from supply, display changes to 14.7955 (approximate) and stays there dropping very very very slowly or almost not at all.

Is this normal?

Have an 8845 at work and it return to it near zero displayed voltage state when probes removed from source.

Wondering about my 8840s.

Thanks,

Steve, the Bonehead

Have an 8845 at work and when v 
Title: Re: Another Fluke 8840 question
Post by: edavid on February 25, 2016, 02:12:44 am
Quote from: BoneHead on February 25, 2016, 01:27:00 am
Browsed / searched for this but didn't find anything.
It's been asked many, many times, but the question is always worded differently, for example:

https://www.eevblog.com/forum/testgear/fluke-8810a-meter-not-rezeroing-after-making-a-reading/msg706371/#msg706371 (https://www.eevblog.com/forum/testgear/fluke-8810a-meter-not-rezeroing-after-making-a-reading/msg706371/#msg706371)
https://www.eevblog.com/forum/testgear/keithley-2015-10v-range-'holds'-previous-dc-voltage-w-leads-disconnected/msg606177/#msg606177 (https://www.eevblog.com/forum/testgear/keithley-2015-10v-range-'holds'-previous-dc-voltage-w-leads-disconnected/msg606177/#msg606177)
https://www.eevblog.com/forum/testgear/keithley-2000-problem/msg732584/#msg732584 (https://www.eevblog.com/forum/testgear/keithley-2000-problem/msg732584/#msg732584)

Quote
Is this normal?

Completely normal.
Title: Re: Another Fluke 8840 question
Post by: deadlylover on February 25, 2016, 02:27:12 am
Totally normal, it's due to the very high input impedance (>1G Ohm).

The 8845 defaults to 10Meg input impedance mode which is why you don't see this effect.  :-+
Title: Re: Another Fluke 8840 question
Post by: BoneHead on February 25, 2016, 03:18:45 am
Thanks edavid, & deadlylover, makes perfect sense; hadn't thought about that; and am used to most of the handheld that have 10 meg ohm input.  I didn't really think it was a problem as both units pass all power up and self tests.

BoneHead