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U3 LV input impedance question


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#1 Gusss

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Posted 22 May 2014 - 01:57 PM

I measure the tiny voltages in plants called action potentials. To do this it is necessary to have an op amp with very high input impedance before the Dac - otherwise the current will follow the path of least resistance into the measuring equipment - so I have an op amp with an input impedance of 1 terraohm (ie electrode > opamp > u3) I notice that on te appendix for the labjack U3 it says that the maximum input impedance is 40 MΩ and max source impedance is 200 kΩ does this mean my op amp cannot be used with the U3 ?

This woul dbe confusing as it is recomended to use the 1040 instrumentation amplifier as it has an input impedance of 10 gigaohms which is also higher thant the spec in appendix A. Perhaps it is more to do with teh output impedance of the opamp/instrumentation amp ?



#2 LabJack Support

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Posted 22 May 2014 - 02:05 PM

See post #24 of topic 6229:

 

https://forums.labja...ge=1#entry22944

 

You plants connect to the input of your op-amp.  This needs to have high input impedance, but usually even more important it needs to have very low input bias currents.

 

The output of the op-amp is what drives the U3 inputs, so the output of the op-amp needs to be 200k or less, which it certainly is.




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