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Ultra high impedance voltage source

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geda7325
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Joined: May 24th, 2012, 2:56 am
Country: Italy

Ultra high impedance voltage source

Post by geda7325 » June 10th, 2012, 11:54 pm

Dear All,

I need to measure a voltage drop (which should be theoretically no more than few millivolts) across a device with an output impedance estimated to be no less than 10^12 or 10^13 Ohms (I'm using a 614). According to Keithley's Low Level Measurement Manual this is actually out of reach, since the input bias current of every electrometer across such a high output impedance source will generate a voltage drop much greater that few millivolts. Is this a correct analysis of the problem?

Thanks in advance,
G.

Dale C
Keithley Applications
Keithley Applications
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Re: Ultra high impedance voltage source

Post by Dale C » June 12th, 2012, 7:11 am

You are near the theoretical limits.
Around 1 T ohms the lowest you can measure is about 1mV.
Here is a thought...
Try using a differential electrometer.
By this I mean use two electrometers.
The high impedance lead of one electrometer on one side of the sample or DUT. And the high impedance lead of the other electrometer on the other side of the sample or DUT.
Look at the application note on this link. Especially figure 6.
http://www.keithley.com/support/data?asset=6584
What is it you are try to measure?
We are in the midst of rewriting the Low Level Handbook. And this might be a good application for differential electrometer discussion.

geda7325
Posts: 7
Joined: May 24th, 2012, 2:56 am
Country: Italy

Re: Ultra high impedance voltage source

Post by geda7325 » June 14th, 2012, 7:11 am

Dale C wrote:You are near the theoretical limits.
Around 1 T ohms the lowest you can measure is about 1mV.
Here is a thought...
Try using a differential electrometer.
By this I mean use two electrometers.
The high impedance lead of one electrometer on one side of the sample or DUT. And the high impedance lead of the other electrometer on the other side of the sample or DUT.
Look at the application note on this link. Especially figure 6.
http://www.keithley.com/support/data?asset=6584
Thanks
Dale C wrote:What is it you are try to measure?
We are in the midst of rewriting the Low Level Handbook. And this might be a good application for differential electrometer discussion.
Well, I'm trying to measure the voltage drop across a S1 photo-tube (PT) without external bias and under different illumination conditions, including complete dark. Under complete dark, I expected to find a voltage drop nearly equal to the input bias current of Keithley 614, times the output impedance of PT, that should be a bit lower than the input impedance of the electrometer. Is this right?

I actually measure a voltage between -.02700V and -.06700V. Namely, when I connect the red lead to electrode 1 of PT and the black lead to electrode 2, I get -.02700V, while when I reverse the connections (red to electrode 2 and black to electrode 1) I get -.06700V. The fact that they are both negative does not concern me since it is maybe due to the sign of the input bias current of the electrometer. On the other hand, I would expect no difference in reversing the connections.

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