How do I decrease the voltage step of my electrometer?

[Swordfish78]

I am running my Keithley 617 Electrometer inside a LabVIEW routine, and it will not allow me to use a voltage step size of less than 10 V. I can go to a larger step, say 20 V, by feeding my count value multiplied by 2 at the VSource input, but feeding 0.1 times the count value to get 1V does not work. It should be noted that this is successful is changing the value of VSource, but the Electrometer and Trek Voltage Source do not acknowlege this smaller step size. Why is this, and is there any way to accomplish what I'm trying to do? Or is it simply a function of the resolution of my Trek Voltage Source (Model 610c)? Thanks.

[mikeporter]

When you say, the setting doesn't work what exactly happens? Are you getting a GPIB error (your error clusters are connected right?)? It the value being set to something else? Is nothing happening?

There could be a bunch of things going on, for example is the count value input an integer value? If so 2 would work, but 0.1 would show up as 0.

First thing is to dig down through the instrument drivers to find the routine that is transmitting the commands to the instrument. Make sure that the command you are sending is the one you think you're sending...

Mike...

[Swordfish78]

No, I'm not getting a GPIB error. The program has been used successfully for quite some time. The only difference now is that I'd like to go to the smaller step size.

I've looked at the Keithley 617 VI itself, and expanded the precision of the VSource value, so that I can see non-integer values are being handled alright.

The symptom that is occurring is the following:

As the VSource value steps in increments smaller than 1, the actual voltage displayed on the electrometer does not step as I'd like it to, but instead fluctuates up and down slightly (out to about the 5th and 6th decimal places).

You may be right that it's simply not happy I'm trying to feed a non-integer value. If that is the case, I guess I can conclude 10V is the minimum
step size I can use?

[Dennis_Knutson]

Can you manually set a smaller step and read it? If you can't, then dig into the instrument manual and see if that's the smallest resolution or you may need to change some instrument setting to give you greater resolution. If you can set the smaller step manually, then there's some bug in your code or the drivers that you're using. In that case, it would help if you would attach the program (including all subVIs) so that someone here could take a look at it.

[Swordfish78]

I will check the manual when I return to work this week, and if that doesn't help I'll attach the program. Thanks.

[Swordfish78]

As it turns out, the Vstep size of 100V is adequate for the purpses of what I'm trying to achieve in this program. However I'd like to make another change that so far hasn't been possible.

The smallest time step that I can apparently use (for stepping the voltage) is 1 second. If I try to use say, 0.1 seconds, the program simply ignores it and uses a time step of 1 second. Is 1 second the minimum that I can use??? This seems unlikely.

It's possible that I have some data representation error somewhere that's preventing me from achieving the smaller step size. I know that the time step, at least, is of type DBL.

I'm attaching a copy of my program. Hopefully it will be beneficial, since it currently contains our instru
ment drivers and may not be executable for you in its current state.

Its main purpose is to ramp the voltage, and measure the correspnding capacitance of an air-gap capacitor, in order to calculate the thickness change of our polymer film samples (same thickness as air-gap).

Let me know if I need to provide any additional explanation. Thanks in advance, I hope you can help!

[Dennis_Knutson]

You didn't include any of the subVIs, so even if I had the instruments, the VI is unexecutable. One thing I did notice is that there is several things going on in the inner for loop - instrument programming and saving to a file. Even though you can set the time to something less than a second, it may take a full second to do everthing. You could try taking the file write out temporarily or look at the instrument VIs to see how long they take to execute. Use the profiler to determine the time for each VI's execution.

[Swordfish78]

I wrote a simple ramping routine to isolate the Electrometer performance from all the calculations. I still was not able to decrease my step size below 1 second.

I removed the Electrometer, and just had Labview do a simple ramp by itself. Sure enough, it can handle as small a time step as I can input.

I conclude the Keithley 617 Electrometer can't step its value any faster than 1 second. Would you agree with this? Or do you know if there a control on the Electrometer that allows for a smaller time step? Thus far I've been unable to locate that control in the manual (or one in the instrument VI either).

[mikeporter]

Given the age of the 617, it's possible that you have hit a limit in the instrument itself that it just can't run any faster...

Mike...

[Swordfish78]

Oh, ok. That makes sense, as long as it sounds plausible to you. We may have a newer electrometer available. I will check its specs. Thanks for your help...