Issue with Aneng AN8008 - Page 1
Forum section: electronics / testgear

author: bloguetronica
author_rank: Frequent Contributor
timestamp: September 03, 2018, 01:21:47 am
content: Hi,I've purchased the Aneng AN8008, but I'm having issues with the mV range. It is a minor nuisance, but the multimeter seems to drift towards the negative end, until it displays "OL" after about one minute with the dial on that setting. Is this normal? I don't wist to buy another unit, but I think I got a defective one.Funny enough, I can't always measure this "OL" voltage with another multimeter, since it will mostly dissipate as soon as I try. Sometimes I see it sitting at 0.3mV, on both multimeters, sometimes I measure it at 1.6mV, and rarely I see a fluctuating voltage of about 120-130mV. This behavior changes by disconnecting and reconnecting the probes. What is consistent is the fact that goes to "OL" or stays like that with the probes fluctuating.This only happens if the problematic multimeter is set in the mV range. The V range works as intended, and doesn't output any voltage that I can measure. I tried to measure the current that the Aneng is outputting in the mV setting, but to no avail. The other multimeter doesn't register anything in the uA range.Kind regards, Samuel Lourenço

author: IanB
author_rank: Super Contributor
timestamp: September 03, 2018, 01:35:59 am
content: What happens if you short the probes together?High impedance voltmeters cannot give a meaningful display with floating probes as they will just measure ambient fields and nonspecific voltages obtained from the local environment.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: September 03, 2018, 02:22:35 am
content: Hi IanB,With the probes shorted, it reads a perfect 0mV. I thought it was stray voltage (don't know the term), but it is just too high. On top of that, I can measure it with another multimeter, although it is not consistent and the results are strange. However, the voltage always seems to go negative and it is as if the multimeter had an internal current source that charged an internal capacitor of some sort (probably it has a few pF of stray capacitance). Only happens at this setting, which makes it even stranger.At the volts range, I get fluctuating voltages in the order of a few mV at most, that can be either positive or negative, but this is normal. I don't see any voltages being produced in this mode. The problem seems to be confined only to the mV range.I'm just wondering if someone has a similar multimeter to see if this is normal.Kind regards, Samuel Lourenço

author: xrunner
author_rank: Super Contributor
timestamp: September 03, 2018, 02:27:13 am
content: Quote from: bloguetronica on September 03, 2018, 01:21:47 amI'm looking at my Uni-T UT139C on the mV range (probes sitting open to the air) drifting to negative at the moment it's at -101 mV and keeps going.*delay*Now at -204 mV ...Go worry about something else.Edit: now at -408 mV

author: bloguetronica
author_rank: Frequent Contributor
timestamp: September 03, 2018, 02:37:42 am
content: Quote from: xrunner on September 03, 2018, 02:27:13 amWow, that is very telling. Does the voltage goes toward zero when you touch both probes at the same time? Probably the chip is the same or very similar.Yes, I agree it is not a big deal, not even a small deal in fact. But I was worried if I got a defective unit and should buy another one, just in case of something else. The current is so tiny that I can't imagine it disturbing the measurements. The mV and the V range are spot on, as far as I could tell, and that is the reason why I bought it.Kind regards, Samuel Lourenço

author: IanB
author_rank: Super Contributor
timestamp: September 03, 2018, 04:11:19 am
content: Yes, my AN8008 does the same thing. On DC mV with no probes inserted the display ramps steadily downwards getting more and more negative without limit.

author: HKJ
author_rank: Super Contributor
timestamp: September 03, 2018, 05:13:10 am
content: With a input impedance above 1Gohm in mV range it is not very strange that there is some drift.

author: IanB
author_rank: Super Contributor
timestamp: September 03, 2018, 05:17:49 am
content: Quote from: HKJ on September 03, 2018, 05:13:10 amBut there is not merely "some drift"If you put the meter on DC mV with no probes inserted the display reading ramps downwards very fast with the speed and consistency of a metronome...

author: bloguetronica
author_rank: Frequent Contributor
timestamp: September 03, 2018, 05:27:57 am
content: Quote from: IanB on September 03, 2018, 04:11:19 amThanks! It seems that I don't have to order another multimeter after all. I though that this could later develop into a problem.Quote from: HKJ on September 03, 2018, 05:13:10 amWas not expecting that, but probably the input stage at that range has some bias current that causes it to drift. This is symptomatic of a constant current source charging a capacitor, except it is a very insignificant current charging a stray capacitance. The linear ramp that is inferred by seeing how the voltage drifts is a telltale sign. That explains it.Quote from: IanB on September 03, 2018, 05:17:49 amLooking on the bright side, I might need a nA current source in the near future.Kind regards, Samuel Lourenço

author: bitseeker
author_rank: Super Contributor
timestamp: September 03, 2018, 05:34:28 am
content: Perfectly normal for high-impedance inputs that are common on lower ranges of multimeters.Here's a fun thing to try: Set it to DC mV, remove the probes, then run your fingers over the input jacks. Your results may vary with humidity, type of plastic on the jacks, etc.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: September 03, 2018, 05:39:41 am
content: Hi beetseeker,Saw no results by doing that, but if I touch the probes, I can get it to drift back to about -20mV (results vary wildly).Kind regards, Samuel Lourenço

author: TurboTom
author_rank: Super Contributor
timestamp: September 03, 2018, 05:58:49 am
content: I find the same behavior with my AN8009. "Shorting" the inputs with 20MOhm results in a reading off approx. -750µV, indicating a leakage current in the ballpark of 30...40pA -- probably nothing to phone home aboutFunny enough, I've got a second AN8009 that doesn't drift at all in the mV range.

author: Brumby
author_rank: Unknown
timestamp: September 03, 2018, 07:42:55 am
content: Quote from: IanB on September 03, 2018, 04:11:19 amSame for me with my AN8008.

author: timgiles
author_rank: Regular Contributor
timestamp: September 03, 2018, 08:34:47 am
content: 30...40pA! You could go to the moon and back with that!

author: bloguetronica
author_rank: Frequent Contributor
timestamp: September 03, 2018, 09:27:49 am
content: And now for a bit of an off-topic, but still about this multimeter. I remember trying to search for the tiny fuses, but had no luck. I've searched on Mouser, Farnell, and also on eBay. It seems I can't find them. I was curious to break one to see if it is sand filled.Anyway, probably they won't be CAT rated for 600V, as the distance between the end caps is in the vicinity of 3mm, even if they are sand filled. It is a bit of a shame, because this multimeter is good in terms of precision. I won't use it on the mains, for sure.Kind regards, Samuel Lourenço

author: TurboTom
author_rank: Super Contributor
timestamp: September 03, 2018, 10:39:19 am
content: Quote from: timgiles on September 03, 2018, 08:34:47 amWell -- my EM-Drive consumes more than that...But to be serious, a jellybean CMOS opamp (LMC7111 for example) is specified at typically 1pA and 20pA as a limit. So the figures for the AN800x multimeters concerned with this problem appear reasonable. Btw, did anyone observe a positive drift or are all the specimen drifting towards the negative rail? It's well possible that the drift is the result of some impurities of the epoxy "blob" used to cover the COB multimeter chip or the PCB itself.