Fluke 8506A multimeter - increasing voltage drift with open inputs - Page 1
Forum section: electronics / testgear

author: SharpEars
author_rank: Unknown
timestamp: October 03, 2014, 12:24:13 pm
content: I just got a used 8506A and when I leave the inputs unconnected and the sensitivity set to either the 1.xxxxxx or 10.xxxxx DC setting, the voltage keeps increasing. It goes up by something like 100 uv per two seconds or so and keeps increasing forever (even after the unit is warmed up). Shorting the inputs does reset it back to zero, but as soon as they are open again the voltage starts increasing. Measurements seem relatively correct once a voltage is connected to the inputs, but this open inputs drift is quite annoying.The weirdness is exacerbated by the fact that this does not happen in the higher voltage modes (i.e., 100.xxx, 1000.xx, etc...). What is causing this drift or is it typical of vintage multimeters like the Fluke 8506A?Here are the specs:www.sy-yq.com/FLUKE/pdf/FLUKE8505A8506A.PDF

author: wiss
author_rank: Frequent Contributor
timestamp: October 03, 2014, 12:37:07 pm
content: It means that your instrument works perfectly fine, it has an input impedance in excess of 10 GOhm, it is supposed to behave like that.

author: SharpEars
author_rank: Unknown
timestamp: October 03, 2014, 12:38:35 pm
content: Actually, this is with the 10 GOhm impedance switched off (i.e., so default 10 MOhm impedance). But why does it behave like that only for the 1.xxxxxx and 10.xxxx settings and not the higher voltage settings?

author: wiss
author_rank: Frequent Contributor
timestamp: October 03, 2014, 01:02:27 pm
content: In that case I guess that the relay that is supposed to switch in the 10 Meg resistor is not contacting properly.What do you get if you measure the resistance of the input?At ranges higher than 10V the 10 Meg divider will always be switched in.

author: SharpEars
author_rank: Unknown
timestamp: October 03, 2014, 01:04:08 pm
content: Quote from: wiss on October 03, 2014, 01:02:27 pmHaven't tried that. What should that give me (I'm hoping that it measures at around 10 MOhm)?

author: Orange
author_rank: Frequent Contributor
timestamp: October 03, 2014, 01:10:37 pm
content: Quote from: SharpEars on October 03, 2014, 12:38:35 pmI'm unaware that you can switch on 10MOhms input on the 1 and 10 V scales on a 8506How do you do that ?Anyhow what you see is normal behavior in the standard mode on the 1 and 10 V DC ranges, mine also does this.

author: wiss
author_rank: Frequent Contributor
timestamp: October 03, 2014, 01:13:37 pm
content: Quote from: Orange on October 03, 2014, 01:10:37 pmYea, I kind of fail to find that switch in the manual as well...

author: SharpEars
author_rank: Unknown
timestamp: October 03, 2014, 01:13:59 pm
content: Quote from: Orange on October 03, 2014, 01:10:37 pmMy bad regarding the switching on of 10MOhm for 1-10 V readings - there is no such switch...If I remember correctly, it has the drift in AVG mode as well (i.e., in every mode capable of 1/10 V DC).One other question for you guys: According to the manual, if I enter the combination RECALL-LO-SCALING (CAL COR), it is supposed to show me the software calibration setting(s) for the current mode. My display just shows me the last voltage reading instead. RECALL-LO-OFFSET (CAL DATE) shows 000000, so that at least seems to show "something meaninful." How does the RECALL-LO-SCALING (CAL COR) work for you guys (i.e., does it work at all)?

author: rstoer
author_rank: Regular Contributor
timestamp: October 03, 2014, 01:24:46 pm
content: AVG mode is still >10G ohms on the lower three DCV ranges. There is no way to switch to 10M ohms on these ranges. This is normal behavior.

author: David Hess
author_rank: Super Contributor
timestamp: October 03, 2014, 01:39:08 pm
content: This is normal behavior for voltmeters which do not have 10 Mohms of shunt resistance on their lower voltage ranges. On some this is a selectable behavior which is nice because it is annoying when you want to use a high voltage probe which requires a 10 Mohm input resistance for proper operation.I have a couple of 3.5 digit voltmeters which support this but they include a jumper to disable it. My HP 3478A is like your Fluke 8506A where >10 Gohm input resistance on the low ranges cannot be disabled.

author: SharpEars
author_rank: Unknown
timestamp: October 03, 2014, 02:27:01 pm
content: Quote from: David Hess on October 03, 2014, 01:39:08 pmSo what is it about >10GOhm that causes the voltage drift?

author: wiss
author_rank: Frequent Contributor
timestamp: October 03, 2014, 03:59:47 pm
content: The voltage of an isolated point is undefined.Anyway, leakage and spontanious ionisation will dominate over ohmic currents.

author: David Hess
author_rank: Super Contributor
timestamp: October 03, 2014, 04:39:59 pm
content: Quote from: SharpEars on October 03, 2014, 02:27:01 pmIgnoring physical leakage currents, the high impedance stage used to buffer the input of the voltmeter has a leakage or bias current associated with it which may be as high as several picoamps or 10s of picoamps. When the input resistance is 10 Mohms, the open circuit offset voltage will only be 10 microvolts per picoamp maximum which is usually low enough to be ignored. With no 10 Mohm shunt restance, the input leakage or bias current charges the input capacitance which is typically from 10s to 100s of picofarads yielding a rising offset of 100s to 10s of millivolts per second per picoamp.Electrometer input stages have lower input bias and leakage currents so they could be 100s to 1000s of times better.

author: SharpEars
author_rank: Unknown
timestamp: October 03, 2014, 07:08:20 pm
content: Quote from: David Hess on October 03, 2014, 04:39:59 pmThanks for the detailed description!

author: MarkPalmer
author_rank: Regular Contributor
timestamp: October 04, 2014, 04:02:38 pm
content: This is one of those things where others see these type of lab meters in my shop, they always look at the numbers are dancing all over the place when they are in DCV autorange and ask why they don't stay at zero. Then you have to try to explain it all to themI just smile and say, "it's the way real meters work."-Mark-