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Using U12 for measuring small currents onto ground

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#1 hp_lj

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Posted 05 February 2013 - 08:37 AM

Dear LabJack community,

I am a complete novice in the use of DAQ devices (and actually don't have a very strong electrical engineering background in general). Your input on the following problem would therefore be highly appreciated:

I have a coopper plate that I use to measure ion currents onto the ground (coming from a nearby coronating wire). I am currently measuring these currents with a pico-ammeter but want to change to the U12 in order to measure several channels simultaneously. The currents I want to measure are approximtaley in the range of 0-20 uA.

The attached figure shows how I have currently connected my setup. I have tried with various resistors (between 100kOhm and 1MOhm) and the results I am obtaining can be made to fit the results I get from my dedicated pico-ammeter. So far so good.

The first problem I am having is that I don't exactly understand the "fitting" that I had to do. I have realised that simply determining the current with I = U/R doesn't quite work, since I need to somehow account for the internal impedance, right? When calculating my current, besides adjusting the "R", I also had to apply an offset to my U readings, in order to match the I values to those from my pico-ammeter. I have so far, through trial and error, derived an equation that works, but now I would really like to understand why?

A second question is whether or not their might be a better setup that leads to less noise. Since I am not interested in instantaneous values but rather averages over several minutes, the noise hasn't been a big problem. But it would of course be nicer if I could have less of it :) Is this possible for such small currents without additional filters, amplifiers, etc.?

Many thanks in advance!



#2 LabJack Support

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Posted 05 February 2013 - 09:23 AM

See Figure 2-4 of the U12 User's Guide, and the text before and after that figure:


Depending on voltage, the signal source needs to source/sink up to roughly +/-100uA of current. Since your source is normally providing just 20uA, I would expect this to have a substantial impact on our experiment. I am surprised that even after calibration you get reasonable numbers.

Our other products (U3/U6/UE9) would do a better job of this, as they have much smaller bias currents, but the real solution is to add a buffer for each channel. You can make your own with a simple op-amp. The basic non-inverting configuration is ideal because it acts like a buffer and also provides amplification:


If you want to buy something, you could use the LJTick-InAmp, or even the LJTick-Divider. Neither plug right in to the U12, but you can use wires to connect them. Figure 3 on the LJTD datasheet shows the LJTD-UNI10V:


You can see that it presents the signal 960 kohms to ground, so you can just connect your current to that and get a signal which is the voltage created across the 240k resistor. Ideally, you (or we) would customize the LJTD to make R1 & R2 much smaller and use R6 & R7 to provide gain.

#3 hp_lj

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Posted 06 February 2013 - 12:40 AM

Thanks for the swift and detailed reply! Very helpful indeed. I would be happy to invest in a U3 if that it is likely to yield better results for my application. Could you give me a recommendation as to whether the HV or the LV is more suitable for my purposes?

#4 hp_lj

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Posted 06 February 2013 - 02:25 AM

So I think I am going to try to implement my own "LJTD". Would you be able to tell me which OP-AMP you use in your LJTD? I read at some other place on your site that you recommend the OPA344? Do you think that one would work for me? Many thanks again for the excellent support!

#5 LabJack Support

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Posted 06 February 2013 - 08:45 AM

Post #3: Not a sure thing that it would work with your signal source, but the best bet is to use a low-voltage analog input (e.g. FIO4) which you get on the LV or HV. I suggest you use a 100k load resistor for your signal, which should give you 2 volts at 20uA. Since your source impedance is greater than 10k, but less than 200k, you want to use LongSettling which is mentioned in Section 2.6.0 of the U3 User's Guide. Post #4: I suggest you use the OPA344 and use Figure 2.6-1 from Section Start with a load resistor like you have now, and the voltage you get from that load resistor is Vin. If you are using a 100k load resistor that creates 2 volts, then you want a gain of 1, so don't install R1 and use something small like 100 ohms for R2. Do you know that your source can generate the 2 volts needed to drive 20uA through a 100k load? Perhaps you should use a smaller resistor and then increase the gain of the op-amp circuit?

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