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NI-9239 how to increase input impedance

the NI-9239 has a listed input impedance of 1 meg. I am working on an application where I need something closer to 10 meg.  Is there a schematic of the input circuitry to allow this modification or will I need to design and build an external instrumentation amplifier?

 

thanks

robert

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You should just use a voltage follower with an op-amp.


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  • I would prefer to not add any external circuitry.  Certainly there is already a instrumentation amplifier in the the input circuit, and a simple modification should allow changing the input impedance.  Do the National engineers look at these discussions and do they provide this information?
  • thanks
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Nobody stops you from voiding the guaranty  😄

Open the housing and have a look. If you have the skills to correctly modify analog measurement gear, analysing the input stage is peanuts.

 

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You will rarely encounter a simple modification that doesn't have other negative consequences in electrical circuits 🙂

I was actually involved in the 9239 project... and based on my memory, the resistor network used on the input stage is a custom bga... so even if you had access to the schematic, and knew exactly what/how to modify the input, you would need to replace a custom BGA component, which I'm not sure if you've ever worked with one of these, but even our best technicians at NI have a very difficult time replacing this component without damaging the boards.

On top of that, I can say with confidence that even if you solve all the other technical challenges, you will likely degrade the accuracy of the device significantly... either due to poor linearity, or noise, or other issues that will come along with not having the impedance that the input stage was designed around.

I hope this is enough information to sufficiently discourage you from modifying the board from its original design. I think your most reasonable options are to implement external circuitry, or to look into a different analog input device that better fits your application requirement.

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That was the conclusion I had come to.  I was hoping a NI guy would say "oh yeah,, we have done a 10 meg version and ..."

 

Instrumentation amplifiers are not a drop kick especially on high impedance differential inputs.  You need to make sure that the input leakage input stage current (pAs) has a ground path, you need to protect against RF induced bias, you have to have over voltage and ESD protection, and you need to make sure that you are operating in a linear range.. presumably  the NI-9239 has done all that,, and now I have to reproduce it.

 

thanks

robert

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If the high input impedance is all you need, you could use DMMs, it depends on the type of acquisition that you want to perform and the specifications that you need.

 

the DMMs can do the job, like the 4065 that has 10M Ohm of input impedance

http://www.ni.com/pdf/manuals/376812a.pdf

 

and some DMMs go up to 10G Ohm, yes, Giga

 http://digital.ni.com/public.nsf/allkb/E76674D886A7850A862574790083BE0F

 

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