EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: paul.gaastra on April 09, 2015, 01:34:03 am
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Hi, We have a uCurrent Gold from the Eevblog shop. On the 1mV/nA range I get an irregular approx. 40kHz oscillation.
This is with a 12V battery in series with a 10M resistor.
Anyone experienced the same thing?
Any fixes?
Thanks
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It will be your setup, most likely picking up noise from somewhere. proper low current measurement is not a trivial thing to do.
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What I would try to do is to add a polyester or polystyrene cap across the combination of R3 and R11 (98K) such that
f = 1/(2*PI*98,000*C) where f is the -3 db frequency and C is the capacitance in Farads. f would be much greater than the frequency of interest.
If not interested in high frequencies the MAX4238 is unity gain stable and reduces the GBW to 1 Mhz. GBW = Gain*Bandwidth.
At low currents, the system could be affected by the tribolectric effect. Motion of the conductors will cause currents to flow.
40 kHz could very possibly be the frequency of a CFL lamp and thus you could be picking up transmissions from that or a computer fan.
Shielded cables have capacitance and can be more prone to oscillations.
Low current measurements are easy to set up, if you know what you have to watch out for. I set up a system to measure below 2 pA without Dave's device.
With these measurements, a shielded fixture is used and a shielded and guarded conductor is used. Specifically Triax cableing which has two shields and a center conductor. 1 meter of this cable with connectors is >$100 USD. The gaurd is "driven" at the same voltage of the inner conductor, so leakage to the shield does not take place. The fixture is guarded/ driven at the center conductor voltage.
Measurements are sensitive to the piezoelectric, triboelectric effects ad EMI and RFI.
What type of 10 M resistor are you using? Think of what might be nearby that operates at 40 Khz. Do the measurement outside with the wires secured or use thick wire so they don't move in the wind.