Amprobe AM-220. Rediculous stray voltage/capacitance? - Page 1
Forum section: electronics / general

author: FenderBender
author_rank: Super Contributor
timestamp: August 28, 2011, 12:41:45 am
content: Hi guys,I have an Amprobe AM-220, as the title suggests. With no probes in or anything, it's reading huge "stray" voltage and capacitance. Resistance, frequency, and current work fine, as you'd expect..but what's with this voltage and capacitance.Capacitance always reads 1.3nF. I can rel it out. And then the voltage. Sometimes if I start it up, it'll stay latched on 350-ish mV. Then if I switch back to it, it will start rising linearly. 0.0, 0.1, 0.2, 0.3, 0.4..etc etc. It'll just keep on going up. I'm confused. I doubt I did anything wrong. Biggest thing I've done with this is put in a fresh set of Duracells.Does anyone else have similar problems? Any possible fixes? Or am I just an idiot?Here's a quick video. Sorry I used my phone.Thanks.

author: IanB
author_rank: Super Contributor
timestamp: August 28, 2011, 12:59:56 am
content: Welcome to the world of high quality meters. It's not a problem, it's how meters are supposed to behave. It indicates they are working properly.The voltage readings are a result of the high impedance inputs picking up stray voltage fields. An ideal voltmeter would have infinite impedance on its inputs meaning it could measure voltage fields in the air.The capacitance readings are due to the stray capacitance of the input terminals. Everything has capacitance, and the meter duly measures what's there.

author: FenderBender
author_rank: Super Contributor
timestamp: August 28, 2011, 01:04:29 am
content: Ah...see I'm still new at this.I thought somehow those things could be like rel-ed out from the factory...I'm still confused how the mV just go up and up and up...

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 28, 2011, 02:27:55 am
content: Rule No1 : At any reading on the display about DC, cross the leads if it reads 000 it is healthy.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 28, 2011, 09:02:49 am
content: 1.3 nF is more than I would expect for stray capacitance. What does it display if the probes are disconnected from the meter?The voltage readings are pretty standard. It is caused by interference from the environment and bias currents of the input amplifier due to the high input impedance. The impedance is > 10 MOhm on volts and > 100 MOhm on millivolts, so nanoamps of noise current can generate the readings you are seeing. All that matters is that the give accurate readings when connected to a source.

author: FenderBender
author_rank: Super Contributor
timestamp: August 28, 2011, 01:30:06 pm
content: 1.3nF was measured without any probes. On the VDC range, it will read 0.00 when I cross the probes. But when I'm on the capacitance range, nothing happens when I cross the probes. Can't rel it out either.Hmmph funny.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 28, 2011, 11:20:10 pm
content: I do not know how Agilent did it, but I almost never user REL on the U1272A .

author: FenderBender
author_rank: Super Contributor
timestamp: August 29, 2011, 12:25:52 am
content: Well it might have something to do with that the Agilent is ~8x more expensive than mine. Not to say that a $50 can't do it right...I'm just curious where the magical 1.3nF is coming from. Maybe it's a software flaw...

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 29, 2011, 12:33:24 am
content: Aside the price range, the point is that now you know that its not a generic phenomenon.And about your specific question, be my guest to contact Amprobe by email.And share with us the reply.

author: IanB
author_rank: Super Contributor
timestamp: August 29, 2011, 12:38:09 am
content: My $50 meter reads about 0.14 nF when the probes are disconnected. If I connect the probes and twist them together the reading goes up to about 0.19 nF.So I guess 1.3 nF does seem a little high--that's 1300 pF. Maybe it is indeed a flaw in the meter.

author: FenderBender
author_rank: Super Contributor
timestamp: August 29, 2011, 12:50:53 am
content: Hmm I might do that. I don't think I could really do anything about it, though. There's no SPI, JTAG, or any type of ISP header to be found...I don't know how they programmed this thing. Whatever it is, it's not obvious to me. This uses a system on a blob...as in it has one of those black blobs of silicon with the die directly on the board. Maybe has something to do with that.I just found the spot where Dave tested capacitance in the video. If you go to 40:37, you'll see that the Amprobe is reading 1.34nF. (The Elenco is also reading 1.32nF, interestingly.) So whatever that means...Either way, I guess I'll just send them an email. Not sure what they would do about it..

author: FenderBender
author_rank: Super Contributor
timestamp: August 29, 2011, 12:52:36 am
content: I mean 1.3nF isn't absolutley huge, and right now, my DMM really just helps me identify parts which I'm too lazy to look up etc., but it's more than you'd expect...especially when there are no probes attached or anything. Did they accidently put a 1nF cap inline with the jack or something?Here goes an email...

author: vk6zgo
author_rank: Super Contributor
timestamp: August 29, 2011, 04:50:37 am
content: Sorry,but 1.3nf strikesmeas being huge!Stray capacitance should surely be in the pf range.If you can "rel it out" it's not so bad.If you can't ,it's fairly useless!I have found that add-on adaptors do a better job than built in capacitance ranges,but"to each his own",I suppose.73,VK6ZGO

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2011, 02:29:18 pm
content: Quote from: vk6zgo on August 29, 2011, 04:50:37 amI agree, 1.3nF is huge parasitic capacitance for a small meter.

author: FenderBender
author_rank: Super Contributor
timestamp: August 29, 2011, 04:33:30 pm
content: I doubt there's actually 1.3nF of capacitance there. It's probably just making a false reading. No email back yet, though I didn't really expect one.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 29, 2011, 04:56:49 pm
content: Yeah, a meter that reads down to 10 pF shouldn't display more than a few 10s or maybe 100 pF with no probes. With probes attached you might get another 100 pF or so of real capacitance if you twist the leads together and you might get more noise.Lower resolution capacitance meters that only go down to 1 nF resolution might have a couple of counts offset, but the actual parasitic capacitance should be negligible.That said, even a lot of high end meters have pretty weak capacitance measurement.

author: FenderBender
author_rank: Super Contributor
timestamp: August 29, 2011, 05:19:05 pm
content: I guess that's why people buy LCR/capacitance meters. Yeah it's still pretty neglible to me. I have a $50 meter. Just by that, you can tell that my work isn't critical anyway.

author: ivan747
author_rank: Super Contributor
timestamp: August 29, 2011, 06:46:30 pm
content: Just use the relative mode. It is functional on mine.The photos are accidentally mirrored. Don't bother

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 29, 2011, 09:27:15 pm
content: Just a tip, never use a mirror for such pictures, it does not work as expected.

author: IanB
author_rank: Super Contributor
timestamp: August 29, 2011, 09:28:53 pm
content: Quote from: Kiriakos-GR on August 29, 2011, 09:27:15 pmIt works entirely as expected, unless you have really strange expectations of mirrors...

author: ivan747
author_rank: Super Contributor
timestamp: August 29, 2011, 11:48:28 pm
content: Quote from: Kiriakos-GR on August 29, 2011, 09:27:15 pmOh no! Sorry. My bad.I was using my webcam. The meters read:01.36nF for the Amprobe0.001nF for the Extech

author: FenderBender
author_rank: Super Contributor
timestamp: August 30, 2011, 01:58:46 am
content: It's not so much of a problem as much as it just pisses me off to see 1.3nF there. Oh well. I guess I should stop moaning. I guess I expected better out of an Amprobe meter.

author: electr_peter
author_rank: Unknown
timestamp: April 12, 2014, 08:55:33 pm
content: It is old thread, but maybe somebody will benefit from this additional post.I have Amprobe AM-240, almost the same meter as AM-220 (I think AM-240 has temperature measurement, otherwise everything is the same).Voltage modes, AC and DC, are a bit random with nothing connected, especially DC mode - that is normal behaviour, it goes back to zero with probes shorted or any external voltage source.What I found interesting is that when I quickly turn on meter (turn knob from off on the left clockwise to DC) meter almost always displays ~320mV on 400mV DC range. I think it has something to do with meter ADC initialisation(or schematic which allows to collect some charge during start up) and consider it a small error. After 320mV reading it goes back to random drift mode, but there is always this strange offset (very consistent) each time I turn on the meter.Capacitor mode shows ~1.4nF with or without probes, so low capacitance accuracy is questionable(higher ranges are fine). Don't assume that resolution (AB.CD nF, 4 digits) always represents accuracy. Dave made a video about thisI wouldn't try to use typical DMM for accurate small cap measurement, but if I did I would use very small/short probes (see topichttps://www.eevblog.com/forum/reviews/fluke-28ii-and-agilent-u1272a-can-do-pf-cap-measurements/) and null function.