06-28-2012 04:45 PM
I was using one NI PXI 4071 for measuring the current, but I want to use two PXI 4071 now to measure current through two channels.
I have attached the code with this post that I use with one card, so can anyone please look at my code and guide me what do I need to change to read current through two PXI 4071.
I will really appreciate your guidelines.
06-29-2012 07:01 AM
I'm assuming you want the samples to occur at the same time. That means you need to use the trigger feature. You need to have both DMMs look at the same PXI trigger signal and start acquisition on that trigger. There is a trigger pallet inside of the NI-DMM pallet. What else do you have in your system? Maybe you can get the trigger out of the DAQ card.
06-30-2012 03:22 AM
I will look in this trigger pallet and see if it helps.
I attached a PDF file it shows the setup of card that I have in my system. Chassis is not shown in this file but both 4071 are in chassis.
Thanks for help.
07-02-2012 05:40 PM
If you go to LabVIEW >> Help >> Find Examples >> Hardware Input and Output >> DAQmx >> Synchronization >> Multi-Device there are examples to help you synchronize two analog inputs on different devices.
07-06-2012 10:56 AM
The easiest method is to just copy and paste the NI-DMM portion of the code, which results in software-timed accuracy (~a few milliseconds at best). With only two DMMs, the code is still manageable; if you have more than two DMMs, I'd recommend creating an array of sessions, and then index through a for loop, opening each session with the same code. Then, once the DMMs are configured and started, you can either read a measurement from each DMM as simultaneously as possible in SW by placing a bunch of reads in parallel and indexing all the session handles from the array, or you can just have another for loop inside your existing for loop that sequentially iterates through each session with the same read command, which will result in each of your measurements being a few ms off (since the calls are made serially). Let us know what kind of synchronization you're looking for and we can comment further.
If you want better syncronization between the two DMMs, then I recommend routing an external trigger to both DMMs (you won't need any external cabling if you can use the PXI Trigger bus). The DAQ board you're using can likely route that trigger. If you need trigger syncronization, let us know and I can whip up some code. Note that even with a hardware trigger, the 407x DMMs are not truly synchronized, but the jitter will be much less and the measurement timing between the two instruments will be much closer together.
Too much information? In short, let us know how synchronized you need your measurements (in milliseconds, etc) and we can delve further.
Have a great day!
07-06-2012 08:26 PM
Thank you so much everyone.
I solved this issue by using two DMMs. I used the same code and just added another DDM for the new "physicall channel".
It is working very well.
My next goal is to have 8 channels doing same work for me but problem is I (my group) can not afford 8 NI PXI 4071, so I plan to use
multiplexing. While, problem with multiplexing is "switching time". Right now I am taking samples at every 5 micro seconds from each NI PXI 4071 which was not possible with multiplexing.
Can anybody help me how to achieve this goal. I will really appreciate your help.
07-09-2012 10:58 AM
Note that depending on the current you're trying to measure, you might need to use a different sense resistance value. For example, if you're trying to measure 1mA, that is only a 100uV drop across the sense resistor; thermal emf and DAQ card accuracy will affect the accuracy of your measurement. In this case, I'd recommend a 100 Ohm resistor, which will yield 100mV with 1mA. Note that the additional series resistance will affect the actual current through your DUT as the series resistance approaches the DUT resistance; you want the sense resistance to be much less than system resistance.
If you're not sure which resistance value to use, post your maximum current and the current resolution you're looking for and we can comment.