Keithley 2400 VISA driver limited resolution

[Blokk]

Hi,

I have a Keithley 2400 sourcemeter, what I use in the +-1mA range, as a current-source. This is the driving part of a PID control.

I use the official VISA drivers, and everything is okey, except that, it seems I cannot use the specified resolution from LabView.

In this low +-1 milliAmp range, the usable resolution is 50 nanoAmperes. In manual mode, everything is ok, I can set for example an output current to be:

-0.245450 mAmperes.

But when I use LabView with the Configure Output .vi from the official llvlib package (i attached it) it rounds for microAmps:

For example the above double value will become: -0.245 mAmperes.

This is really bad for my project, since I need the 50 nanoAmp resolution for the really fine control.

Can someone help me?

Thanks in advance!

[johnsold · Accepted solution]

Change the format string to"%.9f;:" and see if that works.  It appears that the default precision is 6.  Why the original programmer used %s I have no idea.

Lynn

[Blokk]

Thanks very much!

Now I learnt that, I should be always suspicious even with "official" libraries and drivers.

Actually there are two LabView drivers for Keithley 2400, one from Keithley, it is an exe installer type, and the other from National Instruments. I became curious about this bug, and I checked the Keithley VISA drivers, they are okey, they use everywhere double precision variables. 

But if you have a look at the NI Keithley 24XX.lvlib:VI Tree.vi, where is the full list for available VIs, there are many problems I think. At many places (voltage sensing, sourcing, etc) the programmer just used single precision variables to pass information.

I think these problems should be reported toward NI, but I do not know what is the way to do this...

[johnsold]

Unfortunately, many of the drivers are not all that well written.  I find them useful as a starting point but often end up writing my own.

Lynn