Electrical contacts for Femtoamps measurement

I will trust you that you have had problems - though I suspect that you are mis-attributing the cause, or you were also trying to measure microvolt voltages and you are confounding the problems of voltage and current errors. I also trust the results of my own testing, that I did not have problems measuring currents down to > Therefore there is no need to worry any more than usual about RF

Agreed.

Agreed.

By "without gaps" you mean it should be eletcrically connected right? As I said I have used an Aluminum box to keep the assembly inside and close it.

No, I mean "without gaps". Every cable shield has to be attached to the box at both ends, without pigtails. Don't leave anything showing.

You might get lucky like Chris Jones, but don't bet on it.

Cheers

Phil Hobbs

Grin, 90% coax is 10% gaps. An aluminiumized mylar wrap is again your friend.

George H.

Hi Belleman,

If you do not mind I would like to discuss further about it.

My noise is quite messy. I do not see my signal at all. Or that is what it looks like.

The foil is made of toxic metals, so I do not want to break/crush/drill hole in it in my lab. Plus it is extremely thin on the exposed sides, 0.5 mm.

What device do you use to measure current from the i> > > Hello. I would appreciate any response, opinion on the following.

Is it in a grounded metal box? (with preamp inside the box.)

Are you using a lockin technique to try and see the signal?

George H.

I have no separate amplifiers. I am using Keithley 6485 Picoammeter connected through coaxial cables.

The detector part is in a metal box, grounded. but the cable runs long around 15 to 20m to a different room where I have my ammeter.

Yikes. Good luck getting a decent measurement with that rig.

Cheers

Phil Hobbs

Exactly.....

One needs to make a transmitter of sorts with connection at that module to the interested reference..

Any media of transport is sufficient, serial, twisted pairs, buffer amp etc...

But I would think a Hi-Z bridge sensor at the mesure point would be fine with a unity stage after it.

Jamie

Owl: Trouble with you is you don?t use your brains... I?m usin? mine constant!

Churchy: I uses mine only in case of fire.

Mikek

Cute experiments.

BTW, your observations with moving the lid can be explained analytically. You start with an initial charge, q = C V, then you move the lid, changing C, but with the charge unchanged of course. So V must change, V = q / C. As you move the lid away C is greatly decreased, even by many multiples, so V must go up, even by many multiples. Likewise, when you grounded the lid, while it was over a hidden lid with positive voltage, you were starting with -1.5V on the lid, wrt the hidden lid, so as you pulled it away this negative voltage increased, to past -4V as you saw. You could calculate or measure C, and add to the quantitative nature of everything.

On 16 Jul 2015 05:25:42 -0700, Winfield Hill Gave us:

Easy to prove that is the effect by sandwiching an insulator and additional "plates" in the form of foil and more paper separator sheets.

The effects should be more pronounced proving that is where the change is coming from.

Thanks!

Yes, I should have explained that in the video, oh well. I didn't really think much about what I was going to say or do beforehand.

Yes, it might be interesting to try it again with the lid mechanically mounted on a vernier caliper, and measuring the voltage vs displacement. The leakage is low enough that one would have plenty of time to do it, provided the attention span of youtubers would last. I think I have seen something similar as an exhibit in a science museum, but they used kilovolts to charge it, and an electrostatic voltmeter. It is nice to be able to work with just a 1.5V AA cell.

I'm not really sure about what the field does at the edges of a conductor shaped like the tin lid, and I suspect that there isn't any analytical solution (at least within my abilities). There is a free program called "fastcap" that could find an accurate numerical approximation.

I was reminded of this experiment when a colleague at work was showing me a "Kelvin probe", which I now know is a voice-coil linear actuator that vibrates a little metal probe (and its high impedance amplifier) just above the surface of a wafer that is being studied. I think the machine was expensive but I suspect that an old loudspeaker and one of these op-amps and a sound card would suffice to replicate it.

For another project I am reusing an old hard drive voice coil actuator and I require a means of position feedback. I think some sort of capacitive transducer might be good for this, though the plates could be driven at a high enough frequency that leakage current of the circuitry could be relatively unimportant. I am undecided whether to try to design it myself or just use one of the touchscreen chips. Chan built a very simple and evidently effective capacitive position sensor for the galvo servos in his laser projector here:

Chris

Yes very nice... this is a classic freshman year (physics) electrostatic experiment, that in my limited experience (a few years as a TA and one year as the "lab guy", running the labs) never works very well. We did it with big plates and an electrometer. There was always leakage, and stray fields and I'm not sure what all, but it never really gave good data. (In theory the voltage should increase linearly with the distance.)

Getting everything in a nice shielded can may be the trick. I'll have to order an LMC662.

George H.

The LMP7721 should in theory be even nicer - it has low offset voltage too. The problem for me is that it only comes in surface mount packages, and I am not confident of my ability to clean off flux residues, or achieve really low leakage without cleaning them off. One day I will have to do some more cleaning experiments. Last time I tried, the results were poor.

With the LMC662 (or LMC6041 etc also) in DIP packages, I don't have to solder it, and I have had consistently low leakage currents.

Chris

CMOS op amps are good for this, but they suffer from current noise due to differentiated input voltage noise. Keeping the capacitance low is key. (That's one reason that the guy with the 20-metre cables is doomed--that's 2000 pF per cable. A femtoamp will charge that cable at a rate of 0.5 microvolt per second, even if we believe its leakage is that low.)

Charging up a cap and dumping it into the amp all at once is a win. That works with picoamps, but is much harder with femtoamps!

Cheers

Phil Hobbs

Thanks Dr. Philip Hobbs for the wonderful motivation. Much appreciated!