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10 gigaohm input impedance, multichannel DAQ ?


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#1 Gusss

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Posted 23 April 2013 - 12:03 PM

Hi, I am a student measuring tiny electrical impulses in plants (action potentials) Whats the cheapest way of getting a multichannel DAQ (with differential inputs) with 10giga phm input impedance (with an instrumentation amplifier if necessary), I was thinking of using a EI1040 but couldnt find the input impedance....

#2 LabJack Support

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Posted 24 April 2013 - 11:50 AM

With a high-impedance signal source, you need to consider not just the input impedance of the amp, but also the input bias current. In fact, input bias current is usually the more important parameter.

The EI-1040 uses the PGA204. which has an input impedance of 10 Gohms and typical input bias current of 0.5nA. The LJTick-InAmp uses the AD623 which has an input impedance of 2 Gohms and typical input bias current of 17nA. The U6 & T7 have an instrumentation amp built-in which provides an input impedance of 1 Gohm and input bias current of 20nA.

You might need to look for amps with very low input bias currents. You could make your own using an op-amp or in-amp IC, or for a COTS solution perhaps look at amplifiers for pH electrodes.

https://forums.labja...?showtopic=4698

#3 Gusss

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Posted 24 April 2013 - 02:31 PM

thanks a lot ! can you recomend an op amp which has an input impedance of 10gohms or more and a single power supply ? I tried to look on RS website but they have about 3000 and won let me search by impedance !

#4 LabJack Support

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Posted 24 April 2013 - 03:05 PM

You want to search based on input bias current. If the op-amp has low bias currents then it has high input impedance. The reverse is not necessarily true. In the forum topic I linked, the customer mentions the MAX407. Beyond that, try using the parametric search at Analog Devices or TI, or email them to ask about recommended single-supply RIRO op-amps suitable for electrodes and other high-impedance sources.

#5 LabJack Support

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Posted 24 April 2013 - 03:10 PM

The OPA344 mentioned in the U3 User's Guide looks good. Typical input bias current is 0.2pA (input impedance 10^13).

http://labjack.com/s...s-guide/2.6.3.5

http://www.ti.com/product/opa344


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