FLUKE 8840A/8842A: VDC input is not tracking after measurements, bug or feature? - Page 1
Forum section: electronics / testgear

author: Zman
author_rank: Contributor
timestamp: January 06, 2025, 04:49:59 am
content: Hi everybody,I have both FLUKE 8840A and 8842A in my hands and they both show the same strange behavior:when measuring any DC voltage, the readings do not go back to zero with probes flying (source disconnected).Step by step:1) measure any DC voltage source, let's say it is 5V DC from bench PSU2) observe proper readings on the display3) remove the input voltage source (disconnect)4) on both of my units, the voltage reading keeps floating at the same 5V DC level5) now short the probes (input = 0V DC)6) readings go to normal and show close to 0V DCI have never seen this kind of behavior with any other MM (more than 25 different types) and this looks like a malfunction to me.But I just wanted to check with other owners if I am missing something... Is this some sort of weird feature or both of them are failing in the same way?Judging from a quick look at the schematics, any failure in U301, U302, U303 can introduce this kind of behavior, playing tricks with hold-track-measure logic.thanks

author: J-R
author_rank: Super Contributor
timestamp: January 06, 2025, 05:39:54 am
content: 100% normal for those models in the low voltage DC ranges.Some other high input impedance DMMs also behave this way, or drift slowly up or down. They might default to 10M input but have a Giga Ohm option for lower ranges.

author: Zman
author_rank: Contributor
timestamp: January 06, 2025, 09:38:26 pm
content: J-Rthanks for your reply. Are you sure you understand my question?I am not talking about a few mV jumping around zero! I am talking about the unit not tracking the inputs at all.Please read the sequence again, at the step 4, voltage still shows 5V while probes are not connected anywhere. This may happen only if the holding capacitor does not get reconnected to the input or does not discharge at all. It still shows the previous input voltage that is not there anymore.

author: tautech
author_rank: Super Contributor
timestamp: January 06, 2025, 10:22:14 pm
content: Read 2-17. Triggering (P32)https://www.artisantg.com/info/Fluke_8840A_Manual.pdf?srsltid=AfmBOoo46uWs5fnYy4_vt1AfezBnoop0pkZGpH4IOJfPetBINaJxoruP

author: Kleinstein
author_rank: Super Contributor
timestamp: January 06, 2025, 10:30:42 pm
content: The point with the high impedance meters is that the meter also has input capacitance (e.g. a few 100 pF, maybe 1 nF and even much more with anlog filter enabled). As a consequence the open input has a rather large time constant of maybe 100 pF * 100 Gohms = 10 seconds. The actual time constant can be even longer with more capacitance and possibly even higher resistance. The specs are often like > 10 GOhm. This is really a limit for the resistance in the expected resistance is much higher, like the 100 Gohm used in the calculation.So even if the probes are removed the voltage in the input capacitor stays at the old value and can than slowly change from residual input current (e.g. 10 pA range, but different between units).p.s.: with the FLuke 8840/8842 the input capacitance is some 4.7nF and even 660 nF when the analog filter is enabled. So it can hold the charge quite long.

author: Zman
author_rank: Contributor
timestamp: January 06, 2025, 11:38:31 pm
content: tautechare you pointing me at averaging or something else?KleinsteinI just did another test: measured 9V battery and left the probes open for one hour. After this time, it still shows me the same voltage 8.882x as it was an hour ago.Are you saying this is still normal?

author: bdunham7
author_rank: Super Contributor
timestamp: January 06, 2025, 11:53:01 pm
content: Quote from: Zman on January 06, 2025, 09:38:26 pmIt's completely normal with high-impedance meters and it is actually tracking the input voltage--an open circuit is not the same as a short or zero. If the connections are open, how can you know what voltage is there? If you put two high impedance meters in parallel, then they will retain readings or float aimlessly, but they'll do it together. As mentioned earlier, this is all due to very high input impedance, a bit of input capacitance and some bias current from the meter which is typically in the tens of picoamperes.You can just switch to the 100V range where it is 10M input impedance and it will zero out.Quote from: Zman on January 06, 2025, 11:38:31 pmYes. Try doing that with the leads reversed so you display a negative voltage. It probably won't be as stable. The input impedance spec is >10Gohms, but you typically have a much, much higher resistance along with 30pA or so of bias current. Ive characterized the 8842A in my picture and I came up with ~950Gohms and +38pA. You can measure the bias current by putting a 1M resistor across the terminals in the 200mV range and letting it stabilize and then multiply the reading by 106. Whatever the numbers, it will be stable at some point and perhaps yours just happens to be at around 9V. I suppose your bias current could be really, really low but I've always seen 20-40pA on the 8842A models.

author: tautech
author_rank: Super Contributor
timestamp: January 07, 2025, 12:13:22 am
content: Quote from: Zman on January 06, 2025, 11:38:31 pmNo.Do the reading.Measurements are held on the display with relation to trigger settings and can be deleted by pressing the trigger.One must never be scared to RTFM.(Disclaimer, I have never owned or profess to know anything about these DMM's other than what I read in the manual)

author: bdunham7
author_rank: Super Contributor
timestamp: January 07, 2025, 12:17:10 am
content: Quote from: tautech on January 07, 2025, 12:13:22 amYou're probably sending him down the wrong rabbit hole!The EXT TRIGGER function resets at power on, so even if you did bump that it would be a one-time thing.

author: Zman
author_rank: Contributor
timestamp: January 07, 2025, 12:29:50 am
content: QuoteI was under assumption that the holding capacitor gets discharged at some point inside the measurement cycle. Then it gets charged to the new input voltage, thus tracking it.And I thought this is the function of U301C.p.s. What throws me off is that I have never seen such a behavior, even with expensive 6.5 MMs

author: bdunham7
author_rank: Super Contributor
timestamp: January 07, 2025, 12:54:00 am
content: Quote from: Zman on January 07, 2025, 12:29:50 amNothing to the right of U306 is relevant to this issue. [Edit--this is completely true only for the 20V range] It isn't the holding capacitor that is doing this, that would interfere in the regular measurement process. The capacitance here is parasitics (random stuff plus the input protection MOVs) and the 3-pole analog filter which is only engaged in the slow mode. If you just hit the RATE button one time to change from slow to medium, I think you'll see the readings change more quickly open-circuit. (this closes Q304) However, there's no guarantee that they'll go down vs up. Did you try connecting both meters in parallel as I show in my photo?All high-impedance meters behave something like this, although perhaps most aren't quite as stable--you can use this as almost an autohold feature. However, most modern 6.5 digit DMMs don't operate in high-impedance mode by default, you have to select it. Which 6.5-digit DMMs do you have or have you used?

author: Zman
author_rank: Contributor
timestamp: January 07, 2025, 01:20:14 am
content: QuoteSorry, I do not remember - this was a couple jobs back... But they all definitely were branded ones, like HP, Agilent, Keithley or alike.QuoteUnfortunately, I cannot do it right now - I have completely disassembled one unit to replace the faded VFD.Thanks for your time, I will do more testing later.

author: Zman
author_rank: Contributor
timestamp: January 07, 2025, 01:39:55 am
content: Quote from: tautech on January 07, 2025, 12:13:22 amPressing the TRIG button does NOT change the readings on one of my units, neither do the RATE or "external trigger" buttons.

author: J-R
author_rank: Super Contributor
timestamp: January 07, 2025, 03:53:20 am
content: The 8842A definitely is a little bit unusual in how well it retains the value. The question comes up here on the forums every so often. I have three of these and 100% assure you it's completely normal behavior.I have a 34401A and 34461A which both default to 10M input out of the box. I've set them to high impedance mode at power up. The 34401A drifts up, and the 34461A drifts down. Sometimes they drift enough to trigger the next range, which zeroes the reading. Then they drop to the mV range and begin to drift again, starting the process all over again.

author: Zman
author_rank: Contributor
timestamp: January 07, 2025, 05:30:10 am
content: Quote from: J-R on January 07, 2025, 03:53:20 amThanks, J-R! I did tried to search the forum, but could not find the exact answer. I saw people talking only about the near zero drifting, but not about the previous measurements holding...

author: bdunham7
author_rank: Super Contributor
timestamp: January 07, 2025, 05:32:36 am
content: Quote from: J-R on January 07, 2025, 03:53:20 amIt probably bears repeating that the 8840A/8842A are significantly different than most other meters they are comparable to. It is not an integrating meter and does not use dual/multislope methods, it is a sampling meter. It also has some interesting input circuitry, especially around U306. I've never really paid attention before, but as charge accumulates on the input and it counts up, you can change between the 0.2, 2 and 20V ranges and the voltage will be retained. Also, it (mine, anyway) counts up/down the fastest in the 0.2V range, slowest in the 20V range (in absolute terms, like mV/sec) which probably has to do with the fact that the inputs are not connected to directly to the track/hold amplifier in the 20V range. I've never thought of checking the bias current in the 20V range, but I'd not be surprised to see it much lower than the 2V range.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 07, 2025, 09:53:02 am
content: The sampling ADC of the 8840 is not directly effecting how much the meter is holding the old value. The main point is the high input capacitance, expecially with the analog filter enabled. Because of the sampling ADC the input filter is kind of needed for lowest noise.Only few of the other DMMs with integrating ADC have such an input filter that adds a lot of capacitance to the input. Already with the filter turned off there are alread 4.7 nF (C307), which is roughly 10 x what is found with many other meters.It is likely to have less bias and higher input resistance in the 20 V range, as the signal path is different with less switches connected and no more AZ switching pulses to the input. As a downside the 20 V range can have a little extra offset drift (may still not be visible).U301C is not dicharging the sampling capacitor, but used for the auto zero mode to compensate the offset of the JFET amplifier. They actually take care not to discharge the input to keep the input resistance high.