Current Drive with USB 6212

[navneet]

Hello everyone once again.

With USB 6212, the maximum drive current stated is 2mA. Lets say I connect two resistors of 10kOhms and 1MOhms across it. What does this in principle mean?

Does it mean the DAQ will not generate any potential across the 1MOhms resistor?

Am a bit confused.

Will be greatful for your reply.

cheers,

[PHarris]

Hi Navneet,

The analogue outputs on the USB-6212 each have a maximum voltage of 10V and maximum current of 2mA. This means that if the voltage present at the output is placed across a load which is small enough to cause the current to exceed 2mA (from Ohms law, I = V/R), the device will cut out and present an error message on screen stating that the maximum current rating has been exceeded.

So if for example an analogue output was set to it's maximum voltage of 10V and placed across a 10kOhm load, a current of 1mA would be flowing and therefore below the maximum current rating. If however a 1kOhm load was placed across the output with the same voltage, a current of 10mA would flow exceeding the maximum current rating and causing the device to cut out.

Let me know if this helps!

[navneet]

Thanks Paul.

I just got a bit confused as some of my analysis is giving error - problem was with my load itself! It was short circuiting. The maximum possible current is more than enough for my work. However, problem occurs when I connect a large load (1MOhm) and want the current to be near 200uA. I understand I will have to use an external amplifier to do this. Any suggestion how I can simply do this without introducing significant changes what the analog output channel sees.

cheers, 

[PHarris]

Hi Navneet,

If I understand your question correctly, you wish to connect a load of 1MOhm but still have a current of 200uA flow (where as at 10V the current flow would be 10uA). As you said this will require additional amplification of some sort.

Your problem here is that there is going to need to be a 200V potential across your 1MOhm load to provide that kind of current. Could you provide some more details at all on the device you are connecting and why you require a 200uA input current?

All the best.

[navneet]

Hmm. I did not even consider that. It will get more complicated if I do that. I am connecting soil samples of varying resistances inside a box. The maximum resistance of one of these will be 1 MOhm. Others will be much smaller in the order of <100 kOhms. I believe such amplification and later scaling to measure these voltages will create unecessary shifts which will further degrade my analysis. I think I will stick to as it is (with such small currents) as I still am in the linear region even with these small currents. One question though Paul, is there any subVI I can use to improve SNR without causing changes to the measured signal phase. (Stimulus is a sine wave).

Thanks so much Paul. 

[PHarris · Accepted solution]

Hi Navneet,

The only improvement to the SNR you could really make once inside LabVIEW is to apply a low-pass filter to remove unwanted high frequency noise. However digital filtering within software will always induce some constant phase delay between the input and output response due to processing. The input can be delayed whilst the processing occurs and then overlayed on a graph with the processed signal for the purpose of analysis, but this is obviously not realtime. Is this what you meant by a change to the measured signal phase?

I've attached an example vi for you which demonstrates the use of two different digital filtering vi's. The one which is likely to be of interest is the zero-phase filter vi which as you should see overlays nicely in phase with the original signal.

Let me know if this was the kind of answer you were after, I hope it helps.

[navneet]

Thanks a lot Paul and for having a great discussion.

Your patience with me encouraged me 🙂

[PHarris]

No problem Navneet.

Best of luck with your application!