Electronics > Beginners
Beginner level: DIY an accurate picoammeter (measure picoamps!)
NeverDie:
Hi,
The advertisement for uCurrent Gold says:
--- Quote --- "Measure the standby and sleep current of the latest generation of microcontroller and other digital electronics, energy harvesting devices etc, down to the nanoamp (or even picoamp!) level with superb accuracy."
--- End quote ---
https://www.kickstarter.com/projects/eevblog/current-gold-precision-multimeter-current-adapter
Great!
Question #0: What exactly must I do to get the advertised "superbly accurate" picoamp measurements?
I've tried using the uCurrent Gold in conjunction with a 500,000 count DMM, and I'm not getting superbly accurate picoamp measurements.
Background:
I was hoping to use the uCurrent Gold to measure picoamps for some energy harvesting circuitry that's under development: https://forum.mysensors.org/topic/10812/the-harvester-ultimate-power-supply-for-the-raybeacon-dk/190
I recently acquired a 500,000 count DMM (an Amprobe AM-160-A) to complement the uCurrent Gold so as to accomplish picoamp measurements.
However, even after taking anti-static precautions, which seem to have the millivolt readings on my DMM under control, the uCurrent Gold measurements below 1 nanoamp are all over the map. I've grounded my body and have a grounding mat underneath the DUT, the uCurrent Gold, and the DMM and test leads. However, when I move my hands and arms anywhere near it, the picoamp measurements shift around (but stop when I stop moving). The problem is: every time I move, the measurements settle on a different value, so there's no repeatability.
Question #1: Have I diagnosed the problem correctly? Does this phenomena, as I've described it, sound as though static charge is the problem, or is it likely to be something else? Unfortunately, to get the right offset, I have to first measure with uCurrent Gold in shorted mode, and then I have to physically switch the uCurrent Gold to measurement mode, so there's no way I have avoid either touching it or moving near it.
Question #2: Is nanoamp accuracy about as good as I can expect for taking manual measurements using the uCurrent Gold, or would additional anti-static mitigation allow me to get clean, repeatable, manual measurements down to 1 picoamp? If picoamp accuracy is feasible with the uCurrent Gold, what are the minimum prerequisites required to attain it? I need to know so that I can decide whether to spend more on upgraded anti-static measures (if it will work) or else improvise some instrumentation so that I can take the uCurrent Gold picoamp measurements wirelessly, so that I (and my static charge) are nowhere nearby when the picoamp measurements are taken. Would that work, or would even that not be enough? Is there something else I should do/try first?
AVGresponding:
With all the static precautions you're taking, it sounds more like a noisy environment issue (EMI-wise).
Do you have a mobile phone/wi-fi/LED light fittings/SMPS anwhere near the test setup?
NeverDie:
--- Quote from: AVGresponding on May 12, 2020, 04:04:46 pm ---With all the static precautions you're taking, it sounds more like a noisy environment issue (EMI-wise).
Do you have a mobile phone/wi-fi/LED light fittings/SMPS anwhere near the test setup?
--- End quote ---
Nothing nearby. No SMPS. I'm using just a battery for power.
schmitt trigger:
It seems that this could be RF related.
Therefore: a wild, wild, wild idea:
So how about AC grounding yourself?
The normal grounding straps have a 1 Meg resistor in series for safety purposes. How about you put a capacitor in parallel with said resistor? The capacitor value should be calculated to provide a reactance of no less than 100k at your power line frequency for safety purposes.
Otherwise you’ll perhaps require a small Faraday cage.
NeverDie:
Is anyone here able to get superbly accurate, repeatable picoamp measurements using their uCurrent Gold? Has anyone here besides me even tried? It would help my troubleshooting to know if the problem is unique to me or whether others here are either having no problems at all or else either are experiencing or have experienced the phenomena I've described.
I'm having no problems getting repeatable measurements down to about 1 nanoamp of current. It's below that current level where it gets problematic.
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