USB 6008 Rather newbie question

[Mark_Strange]

Okay, first off I do not have much in the way of electronics knowledge, so some of what I am doing may seem ridiculously silly.
Also, I apologise if this has been resolved already, and if so, if someone could give me a link to the proper thread it would be most appreciated.

First a bit of background on what I'm trying to do. I have an electronics device that is designed to apply a potential to a chemical solution between electrodes A and B, and then read the current that passes through electrodes B and C. It cycles the potential between two set values, and by the change in current as a function of potential, you can determine properties of the chemical. (A cyclic voltammograph, or CV,  for anyone who knows about electrochemistry). The device has a pair of leads out so you can read the potential thats being applied, and another pair of leads to give you the current.

I have the potential leads hooked up to the 6008 unit's AI0 + and GND (its in RSE mode, since if I hook it up in Differential mode, I get a 60Hz oscillation in my signal which I assume is from the power to the CV coming from the wall socket).
While this mode seems to give me slightly incorrect voltage readings, I have put a linear calibration in my LabVIEW program to correct for it, and it seems to work fine.
The current leads I have hooked up in AI1 + and -, in differential mode, with a 270 Ohm shunt resistor soldered between the wires.

The LabVIEW program I am running plots current as a function of potential, and as far as I am aware, the code I have works fine.
The problem I seem to have is with the 6008 unit itself. When I am doing a potential cycle with the CV, the current reaches some lower limit and bottoms out.  At first, I thought this was a limit on the amount of current that the 6008 can handle, but it appears that its only about -2mA being read at this lower limit, and I have yet to see the upper limit be reached. (In the same scan, the current went as high as +10mA, and did not get a "flat top").

I have, through process of elimination, determined that its probably something in the 6008 unit, since when I disconnect the leads from the unit, and attach a multimeter to the CV instead, it goes down to -12mA.
(Interestingly enough, with the leads connected, and the pins of the multimeter touching the connection points, the multimeter also bottoms out at the lower limit of about -2mA. This is what leads me to believe theres something electronics-wise in the 6008 that causes this).

Anyway, thats my problem, and again, I apologise if some of the stuff I did sounds completely ridiculous.

Thanks in advance for your help.

[Mark_Strange]

It is I again. I still haven't figured out on my own how to fix my problem of a lower level current cutoff.
Essentially just a bump post, to let people know that I still am seeking help if you can provide it.
Thanks in advance.
 - Mark

[Raajit_Lall]

Mark,

The 6008 does have an input range of +/-10V.  Are you setting the correct range in LabVIEW?  Are you using DAQmx to program this?  It would be helpful if you could post your code. Does it always bottom out at the same value?

Also the input impedance on the analog input channels is 144KOhms, please make sure that your external 270Ohm resistance is not cause some kind of dividing circuit.  All of this information can also be found online here.

Hope some of this helps.

Regards,

[Neil_S.]

Mark,

This behavior does seem very strange to me.  The fact that the signal is bottoming out both on the DMM and the 6008 when the 6008 is attached leads me to believe that the range is not the issue.  As Raajit mentioned the input impedance to the 6008 is 144 kOhms.  I don't see anything that would cause this problem inherently in the 6008 circuitry.  Could you let us know what exactly you have connected to the USB-6008?  For example, are you using the +5 V terminal, or are you using multiple AI channels?  Exactly which terminals do you have wired and what do you have wired to them?

Regards,

Neil S
National Instruments

Message Edited by Neil S. on 06-01-2007 03:29 PM

[Mark_Strange]

I have connected the cyclic voltammograph to the 6008. I am afraid I can't give much more detail than that, as I do not know much about the internal circuitry of the device. I know there is a very large impedance between electrodes A and B (if you read that part in my original post) to prevent current passing between those two, and a low impedance between B and C to allow the current to pass there.

As for hookups, I am using multiple AI channels. AI0 + and GND are wired to the potential leads from the CV, and AI1 + and - are wired to the current leads from the CV.
I wish I could get ahold of the users manual for this particular CV unit, but its about 20 years old and I think the manual is long lost. Otherwise, I could give you more specific information about what is exactly hooked up to the AI terminals.

I will look more closely at the connections on Monday when I am back in the lab, and try a few ideas, then report back here on observed changes.
Until then, I am not sure there is much else you can help me with.

Thanks for your help so far.
 - Mark

[Mark_Strange]

Alright, through a bit of testing I know that the problem is with my shunt resistor.
I tried connecting just straight wires to the terminals without a shunt resistor at all, and the only bottoming out was at +/- 15mA (I assume, there is no longer a reference like the resistor to go from). And this is certainly due to the set range of input.
However, I am not sure that this is a proper way to do it. It seems to work, and as far as I can tell the only drawback is I cannot say with any certainty what my current is. (Which is not that great of a problem for this application).
I am hesitant to continue with this setup, in case its just biding its time before exploding or something, so a professional opinion would be helpful.
 - Mark

[Neil_S.]

Mark,

The 6008 only measures voltage so I am not sure what exactly you are measuring when you just connect a wire between the terminals.  I assume that you are measuring the voltage across the wire but I am not sure how you came up with +/- 15 mA.  How are you calculating this current output.  What value of resistance do you use to make this transistion?

The 6008 will be safe as long as:

1. The voltage between AI and GND doesn't exceed 35 VDC
2. Common mode voltage is kept to a minimum

Regards,

Neil S.
National Instruments

[Mark_Strange]

I am aware that the 6008 only measures voltage, so I am also confused.
The LabVIEW program I wrote uses the express vi for data acquisition, with potential and current channels. I did not change the program at all, so it still thinks I have a 270 Ohm resistor attached, so I assume this is where it is making its calculation.

In the program, I have a graph to plot current as a function of potential. It cuts off at +/- "15 mA", which is where I get the exact values from. But as I said, there is no resistor between the leads, so these values are likely not correct.

The reason I am fairly sure its working properly is the graph does look pretty much exactly as it should, for a plot of I vs V for the particular chemical I am using to test it.

And there is no danger of the voltage exceeding even 5V. The CV device I am using does not have a large range.

As long as there is no danger to the equipment, this seems to work as a solution to my problem. Granted, I cannot trust any of the current measurements, but I'm more interested in the voltage and the shape of the plot rather than the current datapoints.

Thanks for your help so far!