Reading the voltage drop across multiple resistors

[RylonJerome]

HI,
We are creating a PC interfaced IC tester. To do this we need to be able to monitor and record the behavior of the chip in question. The easiest way we have come up with to do this is to place common resistors on the inputs and outputs of these basic devices. By doing this we will be able to read the voltage drop across each resistor, since we know the resistance we can then figure out the current, we measured the voltage and we know all we need to. The problem we are having is reading in multiple channels of data at once. We are using Labview version 7.0 and DAQ PCI-6024E. I have attached the vi file so if anyone can be of any assistance it would be much appreciated. We are beginners here so please be as specific as possible and dumb it down if necessary.
Thanks in advance
Rylon

[RylonJerome]

We were able to read a voltage drop across one resistor in a series circuit but when we attempt to read from another resistor it appears as though they are interfearing with each other and the readings are inaccurate. Any ideas why this would happen????

Thanks

[RylonJerome]

bump

[Sean_C.]

Hello,

How are you configuring this VI to read from multiple channels? Are you setting this up from within the DAQ Assistant? Are you getting any errors when you do this? What kind of interference are you seeing? It might be helpful if you could attach some screen shots to your post.

Please reply to this post and I will be glad to help you troubleshoot the issue.

Regards,
Sean C.

[tbob]

You could be getting bad results due to a race condition associated with your Volt Value local variable inside the case structure. Which occured first, writing to the local, or reading from the variable below? There is no guarantee that one will act before the other. Delete the local and use a wire instead. That way, the case will have to execute first and establish a value before that value is used outside the case structure.

[Ben]

My wild guess:

A high value resistor will appear to be a high impeadence source to the input of the DAQ device. If the interchannel sample time is small compared to the RC time constant of the resistor and input capacitance of the DAQ device (Check spec here) there will not be enough time to full change the input cap to the new voltage while scanning.

THis is characterized by

1) Both voltage look right when measured by themselves.

2) One voltage seems to follow the other.

3) THe problem is less severe when sampling at slower rates.

Done guessing!

Post some code for us to look at! WE do better with pictures.

Ben

[RylonJerome]

The code is up there in the first post. If there are problems with it let me know and I will repost it.

We will try and use smaller resistors and see if that helps but if it doesn't then how can we utilize LabView to measure the resistance, current and voltage and if we can how much interference will each channel give the others?

Thanks for all your help
Rylon

[AnalogKid2DigitalMan]

How are your inputs configured? You may want to consider full differential mode.

[RylonJerome]

We are in full differential mode and have things working well as far as data acquisition. Our problem now is plotting the data. We would like to plot three different curves, the voltages, based on the corresponding temperatures. So we need to make the x axis our temperature and the y our voltage plotting the voltages as they change, in real time with respect to the temperatures.

Thanks for all the help,
Rylon

[RylonJerome]

Here is what we have so far.
Thanks again!