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6487 labview current vs. time measurement

Posted: January 24th, 2016, 10:01 am
by Chess30
I am trying to continuously measure and plot current vs. time using a GPIB-USB adaptor 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?



Re: 6487 labview current vs. time measurement

Posted: January 25th, 2016, 5:25 am
by brian.d.smith
The datasheet and specification are available on our web site. The specification states:

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

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

The hang up is going to be getting data out of the buffer to the pc. The bigger hang up is going to be getting the data through LabView, VISA and WINDOWS.

If you use 0.01 PLC to get fast readings, the accuracy is going to be affected adversely.

If you want very fast and accurate current measurement, you may want to consider a DMM7510.

Re: 6487 labview current vs. time measurement

Posted: January 25th, 2016, 7:52 am
by Chess30
Thanks for the useful comment.
I am measuring very low currents down to pA - nA which is why I am using a - so I can't use the DMM7510. I am also measuring the voltage simultaneously vs. time with a Keithley 2400 (few V range). I used to use a DAQ (quite an old model) to measure voltage directly and connect the - to the DAQ to get faster sampling rates but the ground shields of the BNC connectors on the DAQ were all connected and caused very noisy measurements so I stopped using it.
In general what do you think the best setup for measuring low current and voltage vs time simultaneously with sampling rates of at least 500 S/s but preferably higher is? Do you think if I got a new DAQ this would solve my problems? Is there a current keithley model which can measure voltage and current where the measuremt won't interfere with each other at high sampling rates?

Re: 6487 labview current vs. time measurement

Posted: January 25th, 2016, 8:24 am
by Chess30
I meant the 2002 for the voltage measurement.

Re: 6487 labview current vs. time measurement

Posted: January 25th, 2016, 12:15 pm
by Dale C
Just because the Model 6487 can take reading at the 1000 per second rate does not mean you will have good data at the pA level.
The bandwidth of the analog front end would limit this.
In the nA ranges the bandwidth is around 100Hz.
I would suggest an old Keithley current to voltage converter, Model 428.
This was like a 6487 but no measurements. All it did was convert low current to voltages.
The bandwidth varied depending on the gain. At the 1e11 gain the bandwidth was around 4kHz.
But alas, Keithley no longer has this as a product.
SRS (Stanford Research Systems) has a similar product.
I would suggest to look there for the solution.

Re: 6487 labview current vs. time measurement

Posted: January 26th, 2016, 9:39 am
by Chess30
Are you saying that when I change the range down to nA scale on the -, the sampling rate drops to 100 S/s?
My current program reads at 50S/s even if the current is in mA.
So you don't suggest getting a new DAQ?
What about the DMM 2002? This has an even slower sampling rate, does it have the same problem with bandwidth?

Re: 6487 labview current vs. time measurement

Posted: February 2nd, 2016, 10:22 am
by Dale C
No. the sample rate has nothing to do with the range.
What I meant was the analog circuitry on the more sensitive ranges do not respond as fast.
The analog rise times on the more sensitive ranges are slower than the higher ranges.
Model 2002 would not help. It is the same situation. Lower current takes time to settle.