Digital Multimeters (DMMs) and Precision DC Sources

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What does a NI DMM 40xx look like across its banana inputs before the initialization of a current measurement?

It's not a current meter.

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The thread I started earlier led to my creating what I think is a pretty good response, so I post it here, as well:

 

----- begin paste -----

 

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.

 

----- end paste -----

 

 

 

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<conversation from DM>

 

[*3d0g]

For a NI 40xx DMM configured correctly hardware-wise, prior to initialization of a current measurement using NI DMM software, what is the (input) impedance presented to the external circuit's current path? I cannot find this answer. I think the answer is, between those banana jacks is around 100K Ohms and, hence, no where near being a short. It isn't a current meter. Rather, it is an absolutely awful current meter until the measurement is initialized. Is this correct? If not, then please explain why it isn't.

 

[santo13]

I believe, it is still an ammeter but at a very small current range as default, this is a safety measure as, if you incorrectly connect it across power rails, it will not blow up the fuse or the meter.

 

If you're measuring very small currents, then 100k is still the correct range, just may not be for every application.

 

But, I would not care what the default range is, as a best practice, I will have set up the DMM with the correct range before powering the DUT.

 

<conversation from DM>

 

And why are you not initializing the DMM in the correct current range that you require before you power up?

 

What exact multimeter model are you talking about?

Santhosh
Soliton Technologies

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santo:

 

I tried to give you an extra kudo, but I could only give one.

 

I responded recently (I am so pressed for time) in the other thread started prior to this one. (Use either one -- they're both trying to understand the same situation.)

 

But, I'm using LabVIEW, TestStand (which calls vis), and VoltMeter is a pci-4070, while CurrentMeter is a pcie-4065.

 

Are you saying I should only initialize once at the beginning of the test and that that will keep the CurrentMeter at low impedance throughout the test? (I saw each current measurement as requiring I follow a subvi block-to-block structure. You seem to be saying otherwise, that I only need initialize (run that subvi) once and just leave it.)

 

Thanks for sharing your time with me.

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Yes, you must be initialing any instrument only once at the beginning, reuse the session across all tests and close it only when you're done with using the instrument.

 

You are wasting time by initializing and closing the session every time. Same applies to any instrument configuration, try to eliminate all unnecessary redundant operation to save time.

 

Santhosh
Soliton Technologies

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Just initialize with reset false and leave it that way. Initializing and closing is the safest route. (Is the meter open or not? It doesn't matter if the vi closes it after opening it.)

 

What I'm saying is the reset being false at initialize takes effect with the first read, and if reset is left false, then the meter remains a current meter, even after the close. In fact, it remains a current meter until the PC is restarted. Hence, it's reasonable to do a dummy read just to set up the meter, prior to actually needing it.

 

One thing I haven't done is verify with a scope that the low impedance is a constant, that there isn't a spike, but I don't think there is. ...it was too inconvenient to do, but I really should.

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If initialize and close doesn't significantly affect your execution time, go with it. Typically, in production test where test time is important, it is key to eliminate all sources of redundant time consuming operations.

Santhosh
Soliton Technologies

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If the power off state of the DMM cause a problem, you can use a external current shunt and measure the voltage ..

on the other hand, one could call the power on/off to the load via the DMM a feature 😄

 

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'


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