Reading off on home made uCurrent (osh board rev 5)

[ja421]

(This is a repost from 'repairs' but it seems this is more active for uCurrent)

My 'gold' uCurrent died (I think from over taxing it) -- I emailed Dave & got some tips on what to look at it, and replaced those parts - it was still dead so I decided to build my own (using his board & parts list)  (FWIW: the original unit read 2.5volts on nAmp range -- I replaced a resistor & both max4239's)

For my home made:
Testing went well for mAmp & uAmp testing -- however, nAmp; w/o anything attached/connected - it shows a voltage (between 8 and 12 mv);  I hooked it up to a cheap oscilloscope & it showed some volts (as expected) but as I put my finger close to (but not touching) the current-input posts, the reading fluctuated, a lot.

The uCurrent seems to work properly otherwise, but having a reading w/ nothing connected is a little disheartening.  If I short the input posts, it reads zero as expected. (Shorting with either the switch set to short or using a wire)  Unshorting causes the volts to jump back up. 

The voltage generally is around 10mv and goes up / down from there and only on the nAmp range; the other two range work as expected...

Any suggestions/thoughts on what might be wrong?
Thanks.

[SaabFAN]

I would guess that the OpAmps are set to such high amplification, that they pick up and amplify the voltage across the measuring resistor that is created by the alternating electric-magnetic fields in your room.
Did you get the scope to trigger on the fluctuations? The waveform is probably sinusoidal and the frequency around 50 or 60Hz (depending on the frequency of your power-net.

Also Dave talked about Bias-Currents in one of his videos, which he had to account for when calculating the resistors. Maybe your OpAmps have slightly different Bias-Currents that you have to calibrate out of the circuit with a Trim-Pot.

[ja421]

I would guess that the OpAmps are set to such high amplification, that they pick up and amplify the voltage across the measuring resistor that is created by the alternating electric-magnetic fields in your room.

Well; that will be something easy enough to try; I can kill the power...

Did you get the scope to trigger on the fluctuations? The waveform is probably sinusoidal and the frequency around 50 or 60Hz (depending on the frequency of your power-net.

I didn't use a trigger, but attached is an image showing it.  I have noticed with some sampling ranges that it's just off enough to give wrong readings -- but this was pretty consistent -- I'll replicate it later with more attention paid to getting a good reading.

My bigger concern was the failure to get a proper reading on the low range....

Also Dave talked about Bias-Currents in one of his videos, which he had to account for when calculating the resistors. Maybe your OpAmps have slightly different Bias-Currents that you have to calibrate out of the circuit with a Trim-Pot.

Hmm. I'll have to look for the video; thanks.  I used the identical (part # wise) opamps.  any suggestions on how/where to start with calibrating?   I am a novice here; so I'm learning as I go w/o formal training...

Attached is an image that shows the waveform at nano amp range w/o anything connected.  I will re-test with power off to the building...
The parts highlighted in yellow are part #'s I deviated from specified (basically same part as I can tell; -1 instead of -2); and (digikey) part #'s I used where none specified.

Thanks for your time and ideas.