Hello, my name is Ken of Agilent Technologies.
Today, I'd like to show how to make a high input resistance voltage measurement.
When measuring DC voltage, Digital Multi Meter (DMM) is used in most of the cases.
However, input resistance of typical DMMs is around 10 M ohm when you measure more than 10 volt.
When output resistance of the DUT is large, it cases an error.
The figure on the right side shows this case.
Measured voltage is devided voltage by Rin and Rout.
When Rin and Rout are the same, measured voltage become half of DUT's voltage.
Let me show you how to measure voltage with high input resistance by using Source Measure Unit (SMU).
SMU can force voltage or current and measure voltage and current simultaneously.
In this demonstration, current force and voltage measurement mode is used.
Today, I'll use B2911A Precision Source Measure Unit.
This figure shows DC output characteristic of this SMU, when it is set to I force mode.
From - V limit to + V limit, output current is constant.
Force current value can be set to 0 A,
and this means input resistance value is very high and current flowing into the DUT is very low.
By setting force current value to 0 A and setting + and - V limit to covers voltage range you want to measure,
you can measure voltage of the DUT with high input resistance condition.
Let's use voltage source with 10 M ohm resistor at output to emulate device with large output resistance,
and measure voltage.
This is the connection diagram.
B2961A Power Source is used as voltage source and its high force terminal is connected to 10 Mohm resistor,
then high force terminal of SMU.
Low force terminals of power source and SMU are connected directly.
This is the actual setup.
Let's set up the SMU, which works as a volt meter.
This SMU can be set floating, so I'll set the low termnal setting to floating.
Next, I'll set the mode to current force mode, then source value to 0 A.
Voltage limit will be set to 210 volt.
I'll connect the output and start the measurement.
Next I'll setup the voltage source.
I'll set the force voltage to 200 volt.
You can see the difference between the voltage source and measured voltage is very small.
I'll change the force value to - 200 volt.
You can see that the difference between force voltage and measured voltage is very small.
Let's change the 10 Mohm resistor to 1 G ohm.
I'll connect the output of B2911A and start measurement.
Now, I'll set the force voltage to 200 volt again.
As you can see,the force voltage and measured voltage is very close.
Next, I'll change the force voltage to -200 volt.
The difference between force voltage and measured voltage is very small.
There is a point you must be aware of, when performing this kind of measurement.
If you leave current force mode SMU disconnected from the DUT, like this,
even with 0 A setting offset current woluld be charging up and voltage will go up to + or - V limit.
If you connect your DUT at this moment, discharge current might damage the DUT.
It is safe to change connection to SMU, when channel output switch is turned OFF.
Thank you for watching the video.
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