EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: FrankBuss on February 24, 2017, 05:09:35 am
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I have modified my uCurrent with 3xAAA batteries, maybe I have broken it. Supply voltage is currently 4.4 V. mA and uA range is fine, but nA behaves strange: With 1 uA I can measure 1 mV in the uA range and I can verify the current with my HM8012, but in the nA range the output voltage is 40 mV and it jumps around when I touch the output, sometimes 20 mV. I know, I probably shouldn't do this because of ESD, but the output is stable with the other ranges. Maybe one of the OpAmps is broken and I can order it from Digikey to replace it?
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You might of course have broken it, but I doubt it. It's more likely that you do measure noise. In the nA range, the uCurrent uses a 10kOhm shunt resistor which seems high enough to easily pick up RFI. Best to place the uCurrent *and* the DUT both in an shielded enclosure.
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RFI shouldn't be a problem. My HM8012 has a 1 k resistor for the uA range and doesn't show this problem with the same setup.
But I narrowed down the problem. First I created a test setup without any power supply, to eliminate this as a possible error source, just a 1.5 V battery and a 1 meg ohm resistor. And if I don't touch anything, it shows the right value. But as soon as I touch the positive output or input it changes up to 60%. I verified it with my BM257s and it doesn't have this problem.
I created a short video to demonstrate the effect:
https://www.youtube.com/watch?v=76lvHc5_bi8 (https://www.youtube.com/watch?v=76lvHc5_bi8)
I guess it is me adding some induced mains voltage etc. that causes the problem and now I know this for future measurements, but why don't my other multimeters have this problem? Can anyone reproduce the problem, to verify that this is the normal uCurrent behaviour, and maybe know how to fix it? Usually I don't need to measure fast signals, so maybe adding a capacitor somewhere might help? Or Dave needs to create another "trap for young players" video :)
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I wonder if something is oscillating, can you put a scope on the output?
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You are right, it is oscillating a lot. This is how it looks like when I touch the output:
(http://i.imgur.com/DgicehC.png)
But it is a problem of my measurement setup, I guess because of the cables, because it only oscillates when I'm at the same time near the input with my other hand. I can even adjust the frequency and amplitude when I move my other hand towards the output, very sensitive, like a theremin :)
I added a 100 nF capacitor to the input and the problem is gone. Now the only remaining artifact is when I touch the input, then I can see mains:
(http://i.imgur.com/aT68WB4.png)
But I guess this is to be expected, because enough current flows somehow from my body into it and back with parasitic capacitors. When I touch the output, absolutely no oscillations anymore, problem solved :phew:
And even no need to solder anything, you just need to screw in the capacitor in the banana socket (usually I use banana plugs, so I don't need the holes), just in case someone has the same problem:
(http://i.imgur.com/Kx2Gnlg.jpg)
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Did you put a scope on the uCurrent before you changed to the AAA battery pack? In other words, was there any ringing visible on a scope when using the stock button cell battery?
Also, what are the negative implications of adding that capacitor?
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Did you put a scope on the uCurrent before you changed to the AAA battery pack? In other words, was there any ringing visible on a scope when using the stock button cell battery?
No, I didn't measure it with a scope before. But I guess it is the same, not related to the battery. It is just too sensitive in nA range.
Also, what are the negative implications of adding that capacitor?
If you want to measure AC, the capacitor conducts in parallel with the internal shunt resistor (depending on the frequency), which results in an error. But I think no problem for DC, which I measure most often.
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My issues using ucurrent and scope...
https://www.eevblog.com/forum/projects/ucurrent-ma-range-error-using-oscilloscope-and-function-generator/msg1192329/?topicseen#msg1192329 (https://www.eevblog.com/forum/projects/ucurrent-ma-range-error-using-oscilloscope-and-function-generator/msg1192329/?topicseen#msg1192329)