Using U12 for measuring small currents onto ground
Posted 05 February 2013 - 08:37 AM
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!
Posted 05 February 2013 - 09:23 AM
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.
Posted 06 February 2013 - 12:40 AM
Posted 06 February 2013 - 02:25 AM
Posted 06 February 2013 - 08:45 AM
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