current time measurements using keithley 6487

[kmo]

I want to make current-time measurements for one applied voltage for a diode using labview program.

The vi is attached below.

I am able to get the current measuremnts but I am not sure on how to plot this current against the time.

Any help will be very much appreciated.

[kmo]

Sorry forgot the attachment. Here it is now.

Current_time.vi is the vi that I am making.

I am using the IVsinglescan_transfercurve.vi to take the readings from keithley.

Message Edited by what is this thing on 03-08-2005 08:11 AM

[Johnner]

> I want to make current-time measurements for one applied voltage
> for a diode using labview program

So, you are interested in the current output of the diode as a constant voltage is applied and maintained over a period of time? How fast do you expect the transition to be (what kind of a sampling rate do you think you need)?

It looks to me as if the IVsinglescan_transfercurve subVI you are using is designed to sweep a range of voltages, applying each voltage step at an interval determined by the delay time (ms) input. I suspect the delay time is there to ensure that the current output value has a chance to settle before the reading is taken. This relatively slow voltage sweep is different from what it sounds like you are trying to do, which is to get a series of current measurements at a single voltage, presumably at a pretty high sampling rate.

Keithley has an app note that appears to address the response-time limitations of an instrument like the 6487 when it comes to fast measurements:

http://www.keithley.com/data?asset=13127

I don't know the instrument at all, though, so if I've misinterpreted your goal or the abilities of the 6487, maybe you can reply with some clarifications.

Regards,
John

[kmo]

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@Johnner wrote:
> I want to make current-time measurements for one applied voltage
> for a diode using labview program

So, you are interested in the current output of the diode as a constant voltage is applied and maintained over a period of time?

---

yes, the time is the input. starting from some time for few hours and taking measurements after every few seconds which is measured using delay time.

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How fast do you expect the transition to be (what kind of a sampling rate do you think you need)?

---

I want to sample at high frequency as you already said.

In the program current_time.vi, I get current as output for some single voltage. What i actually want is to get values of current at times. then to plot the current Vs. time for that perticular voltage.

Please do see the current_time.vi, i have mentioned what i want more precisely. I hope i was bit clear now.

[Johnner]

I guess you can use your IVsinglescan_transfercurve.vi subVI to get the job done, but constant-voltage is sort of a special case for it and you'll have to give it some inputs that you have left unconnected so far:

- step size
You need to connect a value of 0 here so that the subVI loops with a constant voltage (the "start" input) instead of using its default step size of 0.01, which will ramp the voltage over time.
- delay time (ms)
If unconnected, this will default to 20 ms. Presumably this is where you wanted to connect your "settle time (ms)" control. This value will help to define the effective sampling rate that you achieve, although the actual rate will depend on how long it takes the subVI to execute the additional code in its "set voltage" and "read current" frames.

The resulting "current data" output array can be connected to a Waveform Graph (no need for an XY graph). If you bundle 0 and the sampling period (in seconds) together with the data array (see attached image) and feed that to the Waveform Graph, you will end up with a plot of current versus time for your voltage input.

The real problem here is that everything is software-timed, so the real sampling rate will be slower than your graph indicates, by an amount that you could estimate if you embedded some timing code in the subVI. Another potential problem is that, for small delay time values, your current-vs-voltage data will be more representative of the settling time of the 6487 than of your actual diode characteristics. See app note I mentioned above.

My two cents,
John

[kmo]

thank you very much for your reply but i still got some problems. To be precise two problems

1)
I modified the vi as you said but there since some of the inputs of IVsinglescan_transfercure.vi are not connected the current_time.vi is giving some error saying that it is failing to run configure_source.vi which is the vi inside IVsinglescan_transfercure.vi. I guess this is happening because i am not connecting the source input to the IVsinglescan_transfercure.vi. Is there any way to get around this??

2)
I want to save the current Vs time data to the file. How can i do that?

Thanks in advance.

[kmo]

I am sorry. I forgot to attach the picture. The picture shows the problems that i was asking solutions for.

Message Edited by what is this thing on 03-24-2005 03:57 PM

[Johnner]

I didn't notice the two different VISA inputs (handleDrain and handleSource). Honestly, I don't understand why you have two different sessions (are you using two different instruments?), so I can't suggest anything other than trying to wire the same VISA session to both of these subVI inputs to see if that might work in your case.

As for the file output, what is the problem? Maybe you need to wire a False into the Transpose? input to get the right structure? Also, I would think you'd want the sampling rate first, so you might want to change your concatenate node so you have that input first, and then the data array.

--John

[kmo]

thanks a lot for your response. Connecting the same instrument handle for both source and drain worked fine.

To save the data in ascii format i connected the input as shown in the attached VI but it doesnt give the correct data. Could you please check it.