why does NI not design zero resistance amperemeter?

[小木虫]

I think zero resistance amperemeter (ZRA) has a zero resistance (is better for current measurement), whether ZRA has a higher measurement precision than NI's DMM? And if, why does NI not design modular instrument based on ZRA? 

Best regards

小木虫

[qchen]

DMM has multiple functions other than rurrent test. you may turn to the specs for parameters.

[Mark_E]

Hi,

I wasn't able to find too much information on the internet about ZRAs, but from what I could tell it would be pretty similar in function to our PXI-4022 6-wire Guard and Current Amplifier Module combined with a DMM. It uses an amplifier instead of a shunt resistor to make current measurements, so the burden voltage can be much lower. I have a few questions for you so that I might be able to clarify a little more:

1. What is your desired application for which you would like a ZRA?

2. What do you mean by "precision"? Do you mean accuracy or resolution? If you could post a link to specifications for a ZRA that you would like to compare to our DMMs/4022 for current measurement accuracy, that would be a helpful comparison. Likewise, you can do as QChen suggested and check our specifications documentation.

Please let us know if you have further questions.

[小木虫]

Hi, Mark,

   Thanks for your answers.

    For Q1: In my test, I usually use device to measure low current (about nA or less), because ZRA has no R in its circuit, then it is the best for my test. From your reply, whether the 4071+4022 can be seen as a ZRA?

    For Q2: I do see little about ZRA as a device to sell. Whether it could say the ZRA is not important or less used than high accurate current measurement device in lab. 

    I have another question: whether a shield cage should be used when a faint current is measured using such as 4071+ 4022 in lab?

    The last question: if no circuit were connected to the 4071+4022, a current could also be measured, is this the noise of environment?  Sometimes these currents have similar trends of real circuits.

Best regards

小木虫

[小木虫]

Hi,Mark,

       I have another question: is there any property node in LabVIEW that can shows the serial number of the DMM connected to your pc?

     Best regards

 小木虫

[Mark_E]

I apologize for not answering your post sooner. I usually just monitor them, and I must have missed that you were never answered.

Can the 4071+4022 be seen as a ZRA?

You can definitely use a PXI-4022 current amplifier together with a PXI DMM to significantly reduce the burden voltage on the circuit under test.  

To avoid loading a circuit completely, use a "hall effect sensor" or "current transducer" that is placed around the current carrying wire and senses the flow of current electromagnetically.  The downside of this approach is that is generally much less accurate than either of the two above options (1% or so), but the big advantage is that you don't have to break your wires, and it doesn't load down the circuit at all. This may not be possible for your application, but thought it would be good to post here.

Should a shielded cage be used when a faint current is measured using a setup such as 4071+ 4022?

Yes. There are lots of things to take into consideration when taking such low level measurements. See How to Minimize Errors for Low-Current Measurements for some good information on things to consider when making these types of measurements. Something else not mentioned is thermoelectric effects (a.k.a. seebeck effect), which are related to temperature differences and the materials used. You should control the temperature of all equipment and use the same material for all connections if possible.

If no circuit were connected to the 4071+4022, a current could also be measured. Is this the noise from the environment?  Sometimes these currents have similar trends of real circuits.

Yes, this is most likely due to environmental effects or internal offsets (caused by bias currents of active devices and leakage currents through insulators within the instrument). Use Offset Nulling to subtract offsets when no current is applied. Reading materials on "low level measurements" can be very informative. The paper I linked above was found by performing this search.

Is there any property node in LabVIEW that can show the serial number of the DMM connected to your pc?

Yes. See the picture below.

I hope that helps.

Message Edited by Mark E on 01-07-2010 09:12 AM

[Henrik_Volkers]

In the nA and lower range my first look goes into direction Keithley, sorry NI ....

A ZRA can be build up with a (compensated) OP AMP and a resistor (or a small C).

However it's tricky, as noted in this paper http://www.national.com/rap/Story/0,1562,5,00.html

[木虫]

Hi, thanks for your answer

    But why you  said it is tricky (sorroy this is not my field)? Whether the ZRA is an ideal instrument but in fact some error will take place during its measurement?

Thanks