miliohm resistance test with Ni9205 or Ni 9219

[WEIQINGQAQ]

Hallo everyone,

i have this two things:

NI 9205 with Dsub AI; 32-Ch +/-10V to +/-200mV 16 Bit AI, Ch-to-COM 9205 and

Universal Analog Input; 4-Ch AI, Ch-to-Earth and Ch-to-Ch Isolated 9219.

What i wanna do ist to test a small resistance (about 1 to 10 mohm).

So i have a rough idea: test the voltage and current.....or make a constant current source and test the voltage on the sample.

I want to know if it works with 9205 or 9219?

Thanks a lot!

[Dharani.R]

Hi,

You can use NI 9205 for resistance measurements as it support 200mV range with ~174uV accuracy. Since resistance mode is not natively supported, you should consider using external current excitation and measure voltage drop across the resistor. This voltage drop should me more than the module accuracy (>174uV). For example: external current source is 10mA, Minimum resistance is 1m ohms. so the voltage drop is 10uV. This means that, you have to increase the resistance value or current value to go beyond ~174uV.

The NI 9219, this module support resistance measurements natively. 4-Wire and 2-Wire Resistance measurement modes. At 1K ohms range you get 0.5 Percent of Reading error as it uses 24 bit ADC.

I hope this helps.

[WEIQINGQAQ]

Hi Dharani,

Thanks for your reply, it helps me a lot.

I'm new about DAQ, and there are still some question confused me.

One ist that with DAQ 9219 4 wire resistance measurement, can i messeure the small resistance like 3 mohm? Cause i see the range is 1 kohm, so 3mohm is also included. Could you please explan me If I measure a 1 milliohm resistance, what would be the approximate error?

The second point is that I see that the 9219 can measure voltage and current, can I use the 9219 to measure both the voltage and current of a circuit and then calculate the approximate 1 milliohm resistance that I need to measure?

Or is there another suitable Daq that would match my needs well?

Thanks again and look forward to your reply.

[santo_13]

As you may find in the datasheet, in the 1k Ohm range, the offset error is quite large, in simple terms, +- 1.2 ohms to +-3.2 ohms. Though you may be able to detect changes in mOhm, you cannot measure it accurately. These are simple DAQ and not intended to replace a DMM for such low measurements.

For your latter question, you can measure voltage and current, they come with their own offset error by the range and you may consume two channels if you want to measure both voltage and current at the same time

[WEIQINGQAQ]

Thank you for your reply.

One more question for you, if I measure the voltage and measure the current in the circuit, can I get a more accurate reading of the resistance by the four wire method and Ohm's law?

Or can you give me some pointers on how to go about measuring such a resistance?

[Dharani.R]

Hi, It seems that on this test of 100 samples / readings, that the NI 9219 had a max value of 986.88 ohms and a minimum of 986.871, so there is roughly a 0.01 ohm error. This means that, it's not best device to measure 1m ohms where you will have 10m ohms error. However, You can increase the accuracy further by reducing sampling rate and averaging the samples.

As you see from the data sheet, the excitation current is provided by the module and it measures voltage drop. So you don't have to provide external excitation while using NI 9219 in resistance measurement mode. This is true for 4 wire measurements but with better accuracy (lead Resistance compensation) Refer to this link to know more - https://zone.ni.com/reference/en-XX/help/370384V-01/dmm/4-wire/

[WEIQINGQAQ]

Hi, thanks for your advice.

I have used NI 9219, the four wire measurement to test my sample. I also compair the result with an old precision voltmeter, and i think it's acceptable. Cause i set the sample size as 10000 and the frequnce as 5000. Through the measurement i find that my resistance ist around several hundred mohms.

For example what i get from Max ist 240.138187mohms, and what i get from Meter is 273.0mohms.

But I have a problem at the same time. What I need to measure is actually a contact resistance, and I am currently making a rough measurement. I use jumper cables for the sample connection. But the connection between the output of the jumper and the sample is a problem, I use an alligator clip to hold them but I don't think this is a permanent solution. (I have marked it in red) Do you have any suggestions or a special kind of such jumper cable with crocodile clips please.

[WEIQINGQAQ]

I also have a question, the 9219 excitation current value is set to 500uA and when I change it it will report an error. I am using the 4-wire method and I think that increasing the current value should help to improve the accuracy. Can you change the current value, I have tried 400uA and 600uA and I get an error. I've tried 400uA and 600uA and the error is reported.

Forward for your Reply and thanks for your help 🙂