two dmms and simultaneous voltage and current measurements

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First, I apologize if this topic is already covered, and I would appreciate a link to the answer if this is the case. I looked a little, but I didn't see my exact issue being covered.

A true current meter looks like a short, while a true voltmeter looks like an open.  In fact, the perfect way to measure both, simultaneously, is to trigger the measurements and have the dmms in use present such impedances to the circuit, simultaneously.

The NI-4065 doesn't seem to look like a true current meter. However, I may be mistaken, as I only did a quick and dirty resistance measurement across the appropriate terminals, where I saw around 100K Ohms.  Yet, my current measurement looks good enough to keep going.  Therefore, I'm concluding that it may look like a true current meter *only* during the measurement; otherwise, it shows 100K-ish to the source's path.  This will cause an error for the voltage measurement side of the measurement using the other dmm.

Do you agree or disagree?  I'm presently saying the NI-4065 (or 4070) is not a real current meter and a real current meter will have to be located for such a measurement. I'd contact NI engineering, but I've been told the forum/community is the better route.  Please help.  Thank you!

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The manual has shown me no solution, either.

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How about this?  (This is when others who know should chime in, please.)

Play with the settling and/or aperture time, using NI DMM properties. (I see this as only a DC thing for right now.)  Specifically, increase the time it takes for the current measurement to complete such that the total voltage measurement takes place while the current measurement is underway.  The voltage measurement causes no circuit loading, but the current measurement looks like a very low resistance as it should while the voltage is being measured.

Yes? No?

Looking like aperture time is the key.

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crickets

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I have dug and dug and dug and dug. Some answers don't exist. I have set aside time to dig for this answer and I have spent that time and I cannot find it. I conclude it does not exist.

[ChrisR.]

Could you sketch out a circuit of how you're trying to take these measurements?  Maybe what you really need is an SMU.

Assuming you want to stick with your two DMMs, you could use one to measure the impedance of the other as it takes a current measurement, leaving your DUT out of it.

[ChrisR.]

Or maybe you're calling reset.vi when you don't need to.  That would put the current-measuring DMM in its default state, which is probably not for current measurement.

[*3d0g]

Thank you for your response, Chris. (I'm finding no clear answers to what I'd think would've been asked by others in the past.)

Here's the circuit I am building/what's happening:

Va ---- (+) CurrentMeter (-) ------- Vb

Vb ------ load --------- VbRet = VaRet

Vb ------ (+) VoltMeter (-) ------- VbRet = VaRet

In the above (node-defined) circuit, one can see I have the current meter in series with the load and the voltmeter across the load.

Therefore, if the current meter is a high impedance prior to its initialization as a current meter, then the load will see very little of the Va source. But once the current meter is initialized, then the load will see the Va source. Hence, I have concluded the NI DMM 40xx makes for a very poor current meter except during the measurement itself.

Am I correct?  It really looks like I'm going to have to apply a shunt closed relay across the CurrentMeter and then open it into the underway current measurement.

[ChrisR. · Accepted solution]

I don't have a 4065, but I have a 4070, and here's what I've found experimenting with it.

Using the Measure DC Current.vi example as written, the amps relay opens then immediately shuts again before taking the measurement.  It then stays closed until the next time you run the VI.

If I wire a "false" constant to the "Reset" input of niDMM Initialize.vi, the problem goes away, and the board stays in current-measurement mode, presenting a low impedance to the current measuring inputs.

So I'd look for some resets hidden in your code.

You don't mention what platform you're on.  PCI?  PXI?  LabVIEW?

The reason this issue exists is because there's room for only 4 properly-spaced banana jacks on the front connector.  So rather than a dedicated "amps" jack, we overload one of the 4-wire ohms terminals.  We considered it safer for the default behavior to present a high impedance.  And resetting restores the hardware to its default state.  The solution is to make sure you don't reset when you don't want to, which should be possible.

BTW, I believe the USB-4065 had room for a 5th banana jack, so I don't think this is an issue on that device.

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Chris:

My setup is using the pcie-4065.

Interesting, so, you seem to be saying it's all about setting the reset input to false, initializing (run that vi), and then leaving it that way, after, through the rest of the testing?  Or did I simplify it too much?

I'm seeing each current measurement as a step by step (subvi blocks) process, starting with initialize, but you seem to be saying, no, just initialize once. ??

Understand there will be current measurements taken sporadically throughout the overall test.  LabVIEW vis will be called upon in via a TestStand sequence. Some of those vis will be measuring currents.

Does this new information change your view?