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High impedance hall measurements

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Joined: January 25th, 2012, 2:56 pm
Country: United States

High impedance hall measurements

Post by claudeahyi » September 30th, 2022, 9:20 am

The 7065 keithley card is supposed to be able to measure very high impedance devices.
However, I noticed that when the impedance is high (>100Mohm) the current source and the ammeter disagree strongly. The current source indicates that whatever current is sourced, is going in but that current is not seen in the device.
The problem is, where is the "excess" current going? If the current source does not see the high impedance, then it is not sourcing enough voltage to force the current into the device. This is noticeable because the source's LED normally blinks when maximum voltage is reached and I don't see that when impedance is high.
note that :
- the card is in high impedance mode (does not make a difference)
- the behavior is correct up to few mega ohm in >2V compliance and up to 200 Mega ohm at 0.2V compliance.
Did anyone see similar behavior at high impedance ?

Andrea C
Keithley Applications
Keithley Applications
Posts: 1769
Joined: October 15th, 2010, 10:35 am
Country: United States

Re: High impedance hall measurements

Post by Andrea C » October 6th, 2022, 3:36 am

How much current are you forcing? I’m guessing nA?
How much difference are you seeing for the measured current?
When you don’t have high resistance sample, do you have agreement then at nA levels?

If you have agreement at low R, but it degrades as your sample R increases, then a significant leakage path is present.
Of course, there is always a leakage path. If it is Tera Ohms, then you don’t notice it so much.
But if it is only G-Ohm, then it can steal noticeable current away from your 100M-Ohm sample.

Cabling and surface contaminations can be the leakage path.

If you do not connect your voltage measuring instrument, how is the current conservation then?

And just to confirm:
For the current source, the voltage compliance needs to be large enough to allow the full I*R drop to occur.
Keep in mind it will see sample + contact R.
The Hall effect card allows only 8 volts, right?

If current source is in voltage compliance (flashing light), then source level is not being achieved and measured value of current will be less than sourced requested value.

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