Freaky Amazing DMM?!

I'm testing a new DMM I purchased, AM-240 by Amprobe. It claims *over* 100Mohm impedance in 400.0mV mode. So I charged a 4.7uF Mylar
capacitor to 36mV DC, and then placed the AM-240 (while in 400mV DC mode) across the Mylar cap. After 25 minutes and 40 seconds it was 35..2mV. That comes to 14Gohms. So I thought it may be due to bias current or offset voltage, and reversed the DMM polarity. Same results. So then I charged the Mylar to 200mV. Same results.
Then, I measured the parallel resistance of my 4.7uF Mylar cap by charging it to 184.8mV, disconnected the AM-240, and 1050 seconds later connected the AM-240 and measured 177.1mV. That comes to 5.25Gohms, which is what I would expect from this capacitor. Actually, for months I've been telling people my guesstimate for this cap is 5Gohms.
Anyhow, what kind of circuit are they using in this AM-240? It appears as if it *resists* change! When it is disconnected it tends to somewhat maintain the DC voltage, regardless of polarity. IOW, lets say it's measuring the DC voltage on the Mylar cap, and it's 180mV. Then one of the leads is removed. The AM-240 DC voltage decreases a bit, not too much, but it slows down, and tends to hang around, say 160mV. If I reverse the polarity, to -180mV, the same thing happens except it hangs around at -160mV. If the AM-240 was measuring say 35mV, and then disconnected, it tends to hand around at oh 20mV to 30mV.
Very interesting DMM. Not sure to like or dislike this.
Thanks for any info. Paul
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Paul wrote:

It could have a guard ring around the input fed from a voltage follower connected to the input. If the gain of the voltage follower is above unity, it could result in something that "resists" change. Difference wrt polarity could be as a result of voltage follower non-linearity.
-- Sue
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I've looked at the specs of ~ 30 DMM's today, include a lot of fluke's, and never seen anything near 14Gohms impedance. Keithley has an electrometer that's probably higher. Most DMM's are around 10Mohms (not gigaohms) input impedance. Don't you think 14 gigaohms is a bit high?
PL
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No, it's a good thing, it's like that by design. Yes, normal meters have a 10Mohm resistor on the input. One ones with "HI-Z" mode remove this resistor and rely just on the input impedance of the FET gate and other circuitry which is there. This value varies a *lot* which is why they typically don't specify it, they just call it "high impedance" mode. E.g. Fluke do not specify the value on their 87 meter, not even a minimum (BTW, hold the Hz button when you power-up to get this mode).
When you need this mode, the input impedance can never be high enough! e.g. when measuring very high impedance circuitry (you can buy Gohm range resistors for example). Actually, even "normal impedance" stuff causes a problem with a 10Mohm input. e.g. you can start seeing errors creep in measuring say >10Kohm stuff.
The cheap Protek 506 & 608 are other meters that have this (not selectable) on the mV range. They spec it at simply >1Gohm.
Dave.
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I'll check out that Fluke. BTW, one thing of significance with my test is that the AM-240 was maintaining the charge on the 4.7uF Mylar! IOW, if I disconnected the AM-240, then the Mylar slowly discharge at a rate equivalent to 5.25Gohms, but when the AM-240 was connected, then the 4.7uF Mylar hardly discharge. Actually, it discharged, but at such a slow rate, equivalent to 14Gohms.
So, what shocked me was that AM-240 help the Mylar retain it's charge. Polarity didn't matter, which rules out bias current or voltage offset. I guess it's a bootstrapping circuit.
Paul
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news:41bfbacc-6f1e-4b44-8aff-

I doubt it, just a high impedance CMOS input circuit (along with the usual protection stuff).
Dave.
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How could it nearly stop the Mylar from discharging. When the meter is disconnected from the Mylar, then nothing is connected to the Mylar, and it discharges at a rate equivalent to 5Gohms. So even if you connect a DMM that has infinite impedance, it's not going to make the Mylar discharge at a slower rate. Somehow the AM240 is *maintaining* the Mylars charge. I'm still thinking about this, lol.
PL
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Another test. I just connected the AM240 (while in 400.0mV mode) to my Keithley. It measured no bias current. The Keithley resolution is 10pA. So that's cool. It has 14Gohm impedance, and who knows how much bias current, definitely less than 10pA.
Paul
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Probably the input of the AM240 has a non linear devices (maybe just the input protection diodes). If I'm right, when you connect a capacitor the non linear device works like a RF detector that charges the capacitor to a voltage lesser than its forward voltage level. The multimeter wires work as the antenna in this case. Try to twist the multimeter wires together and see what happen.
Have fun. Massimo
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The AM240 is not charging the cap per say. So far it has merely tried to *maintain* the caps charge. So far, regardless of the caps charge, or its polarity, the AM240 has tried to maintain the charge.
I've never seen a DMM do this before. Maybe it's possessed. ;-)
PL
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Paul wrote:

If so, that's easy to fix. Just use a Hex Inverter. A 74HC04 should do the job.
I hope this helps...
--
Guy Macon
<http://www.GuyMacon.com/
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Paul wrote:

You're underestimating the insulating power of mylar.
Try soaking the capacitor good and full, then letting it sit, leads in the air, without the meter. Then, after a good long time, measure.
Then you'll see if the meter's been charging the cap or not. Probably not.
Cheers, James Arthur
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Yeah, Mylars are good, but even they are imperfect. I've spent probably too much time testing for dielectric absorption over the years. Although Mylars have hardly no dielectric absorption. I guess there are caps better than Mylars in terms of dielectric absorption. An air gap cap, but how big would it have to be to make 4.7uF?
PL
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Paul wrote:

Sure, mylars leak, and the meter has some bias current too, however small it might be. If you hit the magic voltage the two might even cancel.
Tom Bruhns posted some remarkable polypropylene cap leakage measurements to SED a few years back. Garden-variety 0.1uF caps had 50-year time constants.
http://groups.google.com/group/sci.electronics.design/browse_thread/thread/7a433a7c2b8f072e/d75ac181536b0aa4?hl=en
http://groups.google.com/group/sci.electronics.design/browse_thread/thread/bddb0ddbcf15eef7/626be43dba1b608d?hl=en
Cheers, James Arthur
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In the first link he mentions 50 years for a 0.1uF cap. That comes to 1.6e+16 ohms! Here's the datasheet of my Mylar capacitor or very similar-- don't know the manufacturer of my 4.7uF Mylar --> http://www.panasonic.com/industrial/components/pdf/abd0000ce23.pdf
The spec shows an insulation resistance of >=10 Gohms (20 C, 100 VDC, 60 s), and >=2 Gohms (20 C, 500 VDC, 60 s).
Insulation resistance --> http://www.murata.com/cap/faq/faq03.pdf
I guess it's possible to make a cap with 1.6e+16 ohms, but I would tend to first believe it's due to either dielectric absorption or small signal rectification of electromagnetic signals. As you know, any two atoms forms a junction. There are a lot of impurities in capacitors, thus forming diodes, albeit poor diodes. It's not really possible to have all of the poor diodes counter act each other out, which is probably why a cap, even a good cap can produce a DC voltage in a good electromagnetic field. In the post he says that he did not place the experiment in a shield. From my experience that's normally unnecessary, unless you're near a wifi or radio station, but you never know. It's difficult to say, but my best guess, and it's just a guess, that he was seeing dielectric absorption, and perhaps a bit of DC voltage produced by rectifying electromagnetic signals.
My quick and dirty test of my 4.7uF Mylar showed 5.25Gohms parallel resistance, but I didn't spend much time logging the data. It's probably a lot higher given time to relax. It's possible the insulation resistance would have increased over time.
Paul
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Paul wrote:

I'd say clean the cap carefully with alcohol, soak it at voltage for a few days, then measure leakage.
But if you think it's getting charge from the air, a shielded box would easily answer the question.
Meanwhile, either way you got a nice deal on a DMM with a very high input impedance.
Ain't nuthin' wrong with that.
Cheers, James Arthur
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[snip]
That's true, but once you have the kinks worked out it's not too difficult to measure sources with megaohm impedance. As you know, you have to use low bias current meters or circuits, like an electrometer with femto bias current. If say the meter has 10pA, and the DUT impedance is 100Mohms, then that's 10pA * 100Mohms = 1mV caused just from the meter itself. If the electrometer is 10fA, then it drops to 1uV. Then you have thermoelectric effects unless you use a balanced circuit such as an instrumentation op-amp. After a while of working out the kinks, you can get very stable predictable measurements on high impedance sources.
You probably know all of that. It can take some time to work out the kinks, but such low signal high impedance sources is usually no big deal for a EE who's spent a lot of years working in such a field.
Paul
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The Agilent U1253A also has it. Spec is ">1Gohm". There are quite a few meters I've seen over the years that have it too. I think I even saw it on one of those $10 disposable meters too.
Dave.
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I think the Agilent U1253A typically lists for $450. That's a bit more expensive then the $40 AM240. Do you have model # for the $10 one?
PL
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wrote:> The Agilent U1253A also has it. Spec is ">1Gohm".

Hardly comparing apples and oranges! If you are purely after "the cheapest meter that has a high impedance mV range" then that's a different ball game.

No, sorry. These things come and go like the wind.
Dave.
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