reading with high impedance output (Keithley 2440)

[alihemmati19]

Hi all,

I am using Keithley 2440 and I am going to charge a capacitor and then measure the voltage of that capacitor to see how fast it discharges BY ITSELF. So I need to set the output to High Impedance and measure the voltage.

But my problem is how I can measure while I set the output to High Impedance. I made a vi to do that. But during discharging, Keithley voltage remains the same as charging. So, obviously I am not using High Impedance correctly.

Is there any tutorial for measuring voltage and/or current at High Impedance output setting?

Thanks

[crossrulz]

What do you consider high impedance?  You have to have the voltmeter connected in order to read the voltage.  There is no way around that.  Most voltmeters have an impedance of 1MOhm or 10MOhm.  For most applications, that is considered open enough.

[alihemmati19]

Thanks for the reply.

I think ~MOhm is ok for me. But the problem is that even if I connect to a resistance for example(!) and set voltage (to say 1V) and measure voltage/current for a while, and then set High Impedance output, Keithley still set output voltage to 1V and I measure 0 current/1V voltage. So there is something wrong here for sure.

[Dennis_Knutson]

I don't see where your problem of related to any NI hardware or software. Shouldn't you be checking with Keithley for tutorials?

[alihemmati19]

Hi,

Yes. I looked at the examples that come from Keithley 24XX deriver (vi attached). I suppose that impedance of voltage measurement is high enough so that it does not affect the capacitor that I am going to do measurement on. So I can use Keithley to just measure the voltage.

But, the problem with all "measurement" examples that I've seen is that you need to set an output level to be able to measure it! Why should I need to set output if I want to measure it? 

Thanks!

[nyc_(is_out_of_here)]

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@alihemmati19 wrote:

Hi,

Yes. I looked at the examples that come from Keithley 24XX deriver (vi attached). I suppose that impedance of voltage measurement is high enough so that it does not affect the capacitor that I am going to do measurement on. So I can use Keithley to just measure the voltage.

But, the problem with all "measurement" examples that I've seen is that you need to set an output level to be able to measure it! Why should I need to set output if I want to measure it? 

Thanks!

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Don't you think READING the manual and understanding how the instrument works should be your next step?

[Dennis_Knutson]

You should first setup a measurement manually. If you can do it from the front panel, you can do it programmatically. As noted, reference the manual and Keithley if there problems. I was not referring to the driver examples.