Hello, my name is Ken of Agilent Technologies.
Today I'd like to introduce the precision measurement of resistance which have very high value.
In general, to know the resistance value of some sample,
you will use voltage source to apply a certain voltage to the sample,
with measuring the current value flowing into the sample by ammeter.
Usually, voltage value from power supply is not so precise,
so you should measure by voltmeter.
Then you can figure out the resistance value by "Ohm's law'.
However, when the resistance of the sample is very high,
there are some problems.
One is because of the input resistance of a voltmeter.
Usually the input resistance of digital multimeter is about 10 Mohm
and this limits the value you can measure.
It could be 10 Gohm in some instruments, but then you can only apply 10 V or so.
Another way to measure a high resistance is to use a precision power supply
and not using a voltmeter.
When using this method, low current measurement capability of the ammeter would be a problem.
If the voltage setting is 100 V and the lowest current the ammeter can measure is 1 uA,
then you can only measure up to 100 Mohm.
I'll show you another way to measure high resistance.
The instrument called SMU, Source/Measure Unit has both function of precision power supply and ammeter.
This SMU can measure down to sub pA.
You can perform high resistance measurement with only one channel of SMU.
Okay, let's show you a demo of high resistance measurement.
This is Agilent B2911A SMU.
To measure low current, you should use this type of cable called "Triaxial".
This cable have "guard" structure between signal and ground which minimize leakage current.
The sample should be electrically shielded by this kind of equipment.
This sample has very high resistance,
so I don't have to care about series resistance of the cable.
I choose 2-wire measurement this time.
SMU can perform also 4-wire measurement.
I'm going to use the same value with the slide I showed.
B2911A forces 100 V to the sample.
The resistance value is actually 100 Gohm,
so the measured current would be expected as 1 nA.
I'll force 100 V
and start measuremet by pressing this button.
Okay, approximately 1 nA is shown as I expected.
As a trial, let's show you what happen if you don't use Triaxial cable and shield case.
As you can see, the sample is unshielded and connected to the SMU with just banana cables.
The measured value is 1nA as before,
but you can see more noise on the measured current.
The cable and sample itself is receiving environmental noise.
I recommend you to choose equipment carefully when you perform low current measurement.
In this video I've showed you some tips for high resistance measurement.
Now you understand how you measure, which equipment you need, and what you should take care of.
If you like to have more information,
you can visit our website currently showing below.
Thank you.