triggering and looping with two machines

[screw]

hi,

i have a keithley 6221 current source and a keithley 2182A voltmeter

i'm pretty novice to both measurement equipment and LabView

i need to source current and measure both current and voltage on a resistor as described in the diagram attached

i was told by a Keithly proffessional that the best way to do this is by triggering: namely to make the source trigger the voltmeter and vice versa until some stoppage point while looping this whole thing time after time to produce cycles

how do i do this in reality with actual LabView???

i tried making a sweep followed by simultanoues reading by both instruments with a clock inside a while loop set to be the stepper (measure every 100 ms) and the stopper (40 measurements). this worked but with time inaccuracies that add up to seconds!!

can you please help?! (ANY type of solution is welcomed)

PS: if you want to take a look at my not-working-properly-program, i will gladly post it

thnx in advance!

[Rio C.]

QUOTE (screw @ Jul 29 2008, 10:13 AM)

hi,

i have a keithley 6221 current source and a keithley 2182A voltmeter

i'm pretty novice to both measurement equipment and LabView

i need to source current and measure both current and voltage on a resistor as described in the diagram attached

i was told by a Keithly proffessional that the best way to do this is by triggering: namely to make the source trigger the voltmeter and vice versa until some stoppage point while looping this whole thing time after time to produce cycles

how do i do this in reality with actual LabView???

i tried making a sweep followed by simultanoues reading by both instruments with a clock inside a while loop set to be the stepper (measure every 100 ms) and the stopper (40 measurements). this worked but with time inaccuracies that add up to seconds!!

can you please help?! (ANY type of solution is welcomed)

PS: if you want to take a look at my not-working-properly-program, i will gladly post it

thnx in advance!

Hi screw,

I'm gonna guess that you are using the drivers that you can download from Keithley's website for the Model 6221? This driver represents some top level measurements that you can do with this equipment. To get the best performance you should issue SCPI commands using the VISA read and write VIs. Take some time to familiarize yourself with the manual and develop the remote commands you need. I think for your measurements you can talk solely to the Model 6221. The Model 2182A should be connected to the 6221 with the trigger link cable and the RS-232 cable as shown on page 5-8 of the REFERENCE manual for the Model 6221 (you don't even need a GPIB cable to the 2182A). Also on page 5-51 it talks about differential conductance measurements, which might be similar to the kind of measurement you need. With this mode you can make measurements with intervals as small as 1 ms. Page 5-62 even has an example of the remote commands required for the measurements.

Rio Christensen

[gb119]

QUOTE (Rio C. @ Jul 29 2008, 07:50 PM)

I'm gonna guess that you are using the drivers that you can download from Keithley's website for the Model 6221? This driver represents some top level measurements that you can do with this equipment. To get the best performance you should issue SCPI commands using the VISA read and write VIs. Take some time to familiarize yourself with the manual and develop the remote commands you need. I think for your measurements you can talk solely to the Model 6221. The Model 2182A should be connected to the 6221 with the trigger link cable and the RS-232 cable as shown on page 5-8 of the REFERENCE manual for the Model 6221 (you don't even need a GPIB cable to the 2182A). Also on page 5-51 it talks about differential conductance measurements, which might be similar to the kind of measurement you need. With this mode you can make measurements with intervals as small as 1 ms. Page 5-62 even has an example of the remote commands required for the measurements.

All of this is sound advice - I find the flow charts of the trigger and arm cycles very useful for these instruments in figuring out how to set things up. Essentially you want to have the 6221 emit a trigger signal after it has set the current, have the 2182 wait for this trigger pulse and then have the 2182 emit a trigger pulse after it has doen the measurement for that point to signal the 6221 to move to the next current step.

From memory, the built in modes that hav the 6221 and 2182 work together automatically are rather limited and I'm not sure if you can persuade them to do an arbitary waveform (you can do arbitary pulsed measurements and differential conductance linear sweeps and I think a range of straight and log dc sweeps). If you do have to 'roll your own' then you will need to configure the 2182's buffer to match the number of points that you're defining for the source waveform. Also, whilst it is possible to get the 2182 to talk to you via the 6221, it might be easier to just hook both of them up to GPIB.

In my experience, the Keithley drivers are not very good (but frankly, most manufacturer's drivers aren't very good !) so writing your own is probably the way to go. If I get a chance I'll put my 6221/2182 drivers up somewhere - although they're a bit complex because they're a LabVIEW OOP heirarchy (e.g. Instrument Class -> DMM Class -> 2182 Class etc).

One other comment, you mention wanting to read the current back - the 6221 doesn't actually measure it's own current directly. You could arrange to measure it (in principle) by having a standard resistor in series with current and hook the second channel of the 2182 across it and measure the voltage drop across it to get the real current. On the other hand, you could just trust that the 6221 is doing what you told it to do...