Freaky Amazing DMM?!

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

Nothing new there, many DMM's have selectable "high impedance" or "HI-Z" modes on the mV range. e.g. the Fluke 87.

The input impedance is so high that the small input capacitance maintains a charge when you disconnect. A bit disconcerting when your meter doesn't read zero, but it comes good when you connect a source.

Dave.

It's almost certainly a bootstrapped input amplifier, with the bootstrap gain slightly over unity.

Clifford Heath.

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

Thanks Clifford! I think you nailed it. Anyhow, this is a first for me, and a pleasant surprise to learn of this.

I have no affiliation with Amprobe, but as far as inexpensive ($40) DMM's go, this one seems like a gem. I was going to take it back today at Frys Electronics for the PM51A because it claims 1Gohm impedance, while the AM-240 only says > 100Mohms. Hmmm, 14G is far greater than

Paul

BTW, here's a bootstrapped input amp circuit -->

Paul

What's it's input capacitance?

RL

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.

I don't see it listed on the spec sheet, but I've seen a lot of other similar Amprobe meters that are around 30pF input.

Paul

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

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.

Most likely an R-C circuit (maybe 10M and 1uF) going into a CMOS analog circuit with almost no leakage typically (at room temperature).

I doubt it, just a high impedance CMOS input circuit (along with the usual protection stuff).

Dave.

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

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

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.

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 =3D 1mV caused just from the meter itself. If the electrometer is 10fA, then it drops to

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

I have a few $3 DMM's from Harbor freight, but they're 2Mohm impedance in the 200mV setting.

PL

Another test. I just connected the AM240 (while in 400.0mV mode) to my Keithley. It measured no bias current. The Keithley resolution is

Paul