continuous current-time measurement with keithley 6487 picoammeter

[flb30]

I am trying to continuously measure and plot current vs. time using a GPIB-USB adaptor with a keithley 6487 picoammeter and a labview program (attached). When the program ends it then saves all the acquired data. I am trying to measure this at th highest possible sampling rate by using the buffer and each current measurment comes with a timestamp.

The first problem is that the time values seem to have 1E+9 added to them, does anyone know why and how I get rid of this?

The second problem is that the data acquisition is very slow even using the buffer. I essentially want to use the - like an oscilloscope. I want to continuously acquire current with the timestamp value and plot at a sample rate of 2000 S/s. Is this possible? What do I need to change in my program to do this? If this sample rate is not possible with a timestamp, what is the maximum sample rate achievable?

Thanks,

Chess

[GerdW]

Hi flb,

I want to continuously acquire current with the timestamp value and plot at a sample rate of 2000 S/s. Is this possible?

The first you should do to answer such questions is RTFM!

Even the smallest spec sheet offered by Keithley is answering your question!

time values seem to have 1E+9 added to them, does anyone know why and how I get rid of this?

I guess this also is explained in the manual of your device…

[flb30]

This is the spec, of course I have read the manual.

READING RATE:
To internal buffer 1000 readings/second; note 1
To IEEE-488 bus 900 readings/second; notes 1 and 2

Notes:
1 0.01 PLC, digital filters off, front panel off, auto zero off.
2 Binary transfer mode. IEEE-488.1.

I don't get anywhere near this rate using my labview program and that's what I want help with.

I can't find anywhere in the manual which explains why I am getting 1E+9s added on to my time loop every I think it is a programming problem.

I hope you can help this time.

[Dennis_Knutson]

I can't view your VI at the moment. Can you attach a snippet of the block diagram? I also hope you are not using a timed loop on a regular Windows pc. A timed loop should only be used on a real time os.

[flb30]

I am not using a timed loop and I am using a windows pc.

This is the first labview program I have written so be as clear as possible thanks.

[Dennis_Knutson]

So, how many measurements are you actually acquiring? Can you attach a screen shot of the VISA Read string?

I'm not sure you have a LabVIEW problem. You might want to check with Keithley for the exact set-up to get the stated maximum. I would recommend that you do your initial debug in MAX before writing any code.

[flb30]

The 3 screenshots show the visa read bit. I have attached the format data vi screenshot too.

My main question is about labiew not the keithleys. When I run the program, the first loop works fine but then the time values suddenly start adding 1E9 onto every loop. Why does this happen?

The number of points I am reading is set by how many buffer points I choose to select.

Also the way I plot the data, after a set of buffer points is plotted, the next set of buffer points plots from the first point in the previous set of points instead of the last. How do you change this?

[GerdW]

Hi flb,

My main question is about labiew not the keithleys. When I run the program, the first loop works fine but then the time values suddenly start adding 1E9

onto every loop. Why does this happen?

If you see such behaviour the first you should do is start to debug the offending code! Have you? Got results?

I guess it happens in the part of your VI where you generate the "x axis" time stamp values. You subtract two timestamps, coming from a shift register. Are these (always) useful values?

[Dennis_Knutson]

You did not attach the contents of free string line I asked and your original question was that you could not acquire at a desired rate - an instrument set-up problem.

[David_NMRSA]

hey mate, just having a quick look here I notice that the Keithly 6487 picoammeter provides resosolution down to pA (even fA!!). This implies a 1e-12 value, if measured in Amps.

If you are getting 1e9 on your results, my guess would be that you are measuring Currents of the order of mA. Does that sound about right, based on the type of measurements you are doing?

Just a guess, mind you...

I'll  have a look at the rest and see if notice anything else.