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White noise 6220 DC CURRENT SOURCE

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ALeroux
Posts: 2
Joined: April 7th, 2015, 5:52 am
Country: France

White noise 6220 DC CURRENT SOURCE

Post by ALeroux » April 7th, 2015, 6:08 am

Dear Sir,

I am trying to use 6220 DC current source in order to supply 10mAmp through stain gage.
As deformation level to measure is under one microdef, I am facing with a resonance of the background noise
of the generator around 6 - 6.5 Khz.
Where does this ''bump'' in the background noise come from ???
I have been blocked in my experimentation for weeks, France support can not help me
Thanks for any help !!

Yours,
Ir André Leroux
Snecma, Safran Group.
keithley_white_noise.jpg
keithley_white_noise.jpg (50.75 KiB) Viewed 11705 times

Norbert W
Keithley Applications
Keithley Applications
Posts: 227
Joined: October 21st, 2010, 7:18 am
Country: Germany

Re: White noise 6220 DC CURRENT SOURCE

Post by Norbert W » April 7th, 2015, 7:04 am

Hello,

in order to better understand your setup let me ask several questions:
- please describe your bridge in more detail (I would guess a Wheatstone arrangement,
but is it a half bridge, what resistor values &c. - the idea here would be to see if a filtering capacitor on your
supply connections is feasible)
- are you forcing 10 mA in the 10 mA range?
- could you supply the same noise measurements when the 6220 (HI) is not connected at all?
(this tends to look for common mode issues in your measurements as the bridge should attenuate noise levels
from the current source...)
Thanks for your efforts in advance.

ALeroux
Posts: 2
Joined: April 7th, 2015, 5:52 am
Country: France

Re: White noise 6220 DC CURRENT SOURCE

Post by ALeroux » April 7th, 2015, 7:39 am

Hello Norbert,
fist of all waww for your fast answer and thank you.
It is a quarter bridge, thus a simple 120 ohm resistor + 100 ohm of wiring.
I was needing high accuracy low noise DC generator.
In parallel are connected: a. the generator, b.the resistance, c. the acquitition system.
If a capacitor is installed, it would shut down the signal. Maybe an self could be added
in serial with the generator but it does not make me sure about power going on and
off due to the possibility of surges.
I have purchases two 6220 and I did try both. I've found the same symptom almost
at the same frequence. I did try a ''regular'' resistor. Same problem.
I am forcing the current in the lower range (10 mAmp)
Same problem with a second different acquisition system.
Maybe a DC generator is not suited if higher than DC frequency matter.
Did ever someone do AC analysis of a DC generator ?
Could Keithley do the test Generator + 120 ohm resistor + spectrometer. It is a very direct test.
What puzzle me is that this ''bump'' is not spike like generated by some digital clock but it looks
like first order resonance overshoot.

Thanks again.

Yours,
André.

Norbert W
Keithley Applications
Keithley Applications
Posts: 227
Joined: October 21st, 2010, 7:18 am
Country: Germany

Re: White noise 6220 DC CURRENT SOURCE

Post by Norbert W » April 9th, 2015, 7:48 am

Hello,

I tried to duplicate the issue you described - without real success as I could not see a bump at 6 to 6.5 kHz.
Just given your arrangement (bridge type) an additional attenuation should happen close to balance.
Therefore I would be interested in your measurement results with everything connected but the 6220
power switch being set to OFF (so the 6220 effectively not working at all). This should help to identify the cause of the problem you see - I still doubt the
6220 is to blame.
Would appreciate a screenshot as you made before. Please also mention the scaling used.

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