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Specification of NI ELVIS II+ DMM

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Specification of NI ELVIS II+ DMM

Hi,

 

Recently I'm using NI ELVIS II+ for sensor characterization due to resistance measurement up to 100Mohm.

However, I found the specification stated 100Mohm measurement can be done using max test voltage source of 5V and internal current source of 500 nA? By using Ohm's Law, 10Mohm turned out to be the maximum measurement? 

 

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Regards,

Yung Sin Chong

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Re: Specification of NI ELVIS II+ DMM

 

Hi Sir Yung Sin Chong,

 

Looking at the NI Educational Laboratory Virtual Instrumentation Suite II Series (NI ELVIS II Series) User Manual, on page A-14, it might be a typographical error on the Elvis II Specification Manual. That should have been 10Mohm.

We will check that with our Product Engineers.

 

Thanks and Regards,

Jvi Pyl Pasia

Application Engineer

National Instruments

 

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Re: Specification of NI ELVIS II+ DMM

Hi Pasia,

 

Thanks for your reply. It will be great to have confirmation with them. I am looking forward for the good news from you.

 

Thank you.

Best Regards,

Yung Sin Chong

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Re: Specification of NI ELVIS II+ DMM

Hi Sir Yung Sin Chong,

 

I have consulted this issue with our Academic Team and found out that we made a wrong conclusion with the specification.

The specifications are correct according to them. There is an internal shunt resistor used to allow us to measure up to 100MOhms. That might clear it up.

 

Thanks,

Jvi

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Re: Specification of NI ELVIS II+ DMM

Hi Jvi,

 

Thanks for your confirmation. However, it will be great if you can explain in more detail about the internal shunt resistor and internal excitation current 500nA which intialize for 100Mohm measurement in VI?

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Re: Specification of NI ELVIS II+ DMM

Hi Sir Yung Sin Chong,

 

The internal shunt resistor is not involved in the programming. It is on the circuitry of the ELVIS DMM and it might be complicated to explain. You can just follow the same procedure of measuring through the ELVIS DMM.

 

Thanks and Regards,

Jvi

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Re: Specification of NI ELVIS II+ DMM

Hi Jvi,

 

It is not a problem for me in understanding the working principle of electronic circuitry because I'm a EE undergraduate. If it involves the P&C issue behind the design and if possible I hope you can drop me a message to discuss personally. 

 

Thanks & Best Regards,

Yung Sin Chong

 

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Solution
Accepted by topic author sin_chong
08-29-2016 07:40 AM

Re: Specification of NI ELVIS II+ DMM

Hello,

 

The NI ELVIS II DMM has a permanent 11 MOhm input resistance. For resistance measurements, the Device Under Test (DUT) is assumed to be in parallel with this 11 MOhm. DUT resistance is calculated from this nonlinear relationship by measuring the test voltage considering calibrated excitation current and parallel resistance. 

 

For example, a 10 MOhm DUT measured in the 100 MOhm range with a 500 nA excitation current would show about (11 M || 10 M) * 500 nA ~= 2.6 V.

 

With a 100 MOhm DUT, the test voltage would be about (11 M || 100 M) * 500 nA ~= 4.95 V... right up against the 5 V limit.

 

With an open, the test voltage is (11 M) * 500 nA ~= 5.5 V which is greater than the 5 V limit and is detectable as "OPEN" in software.

 

The NI ELVIS II DMM design was leveraged from another DMM whose input resistance was selectable (11 MOhm or >10 GOhm). In that case, the 500 nA excitation could be used in conjunction with the 10 GOhm input impedance to give a 10 MOhm resistance measurement range. Since the NI ELVIS II DMM's input resistance is fixed, we didn't offer a separate 10 MOhm range.

 

I hope this is helpful,

 

Charles Y.

National Instruments

 

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Re: Specification of NI ELVIS II+ DMM

Hi Charles,

 

Thank you for your detailed and clear explanation. It was very helpful and understandable.

 

Best Regards,

Chong Yung Sin

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Re: Specification of NI ELVIS II+ DMM

 

Thanks for your help Charles.

 

Hi Sir Yung Sin Chong,

 

As per our Academic Team:

The shunt resistor is in parallel with the load measured, therefore decreasing the overall resistance. The value is compensated for in software without the user needing to account for it.

 

I think that clears it up.

 

If you have other questions, you can contact us at asean.support@ni.com.

 

Thanks and Regards,

Jvi

 

 

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