04-12-2012 02:38 PM
I use a NI4461 card with 2 input and 2 output channels. I have a signal generator with a 100Vpp 125Hz signal that I would like to characterize using labview (FFT, amplitude etc).
As the input limits on the NI4461 are +/-42.4V, I was hoping to use a 10X probe, similar to a scope.
However I find this not working correctly as the probe attenuation does not seem to be a constant 10x. Although my scope (with a second probe) reads 100Vpp, my NI4461 only detect 82Vpp.
Am I missing something? Are there other solutions to read >42.4V signals on the 4461 than using a probe?
I checked and swapped probes and they both work find on the scope, just not on the 4461. Is there a special probe that has a good impedance match for the 4461 that one can recommend?
Solved! Go to Solution.
04-13-2012 05:29 PM
First, when you say your 4461 only detects 82Vpp, do you mean it only detects an attenuated signal that corresponds to 86Vpp?
The 4461 has a few built-in gain settings, so it's possible that it's attenuating the signal itself to some degree. These can be found on page two of the NI 446x Specifications - be sure to check if you've enabled this feature.
In terms of a solution, you may not be able to properly measure this voltage with this card. I will look into other options for you, and should have some more information early next week.
Have a great weekend, Peter!
04-16-2012 11:06 AM - edited 04-16-2012 11:09 AM
Thanks, I accidently opened two threads.
I would very much appreciate any help/suggestions to measure 100Vpp 125Hz signal with the NI4461.
The signal was not clipped when I measured 82Vpp. It actually measured 8.2Vpp with the 10X probe but in Labview I multiplied the signal by 10X.
According to what I would expect, it should have measured a 10Vpp that, with the multiplier, would have been the 100Vpp equivalent.
However, the probe seemed to attenuate the voltage signal measured. This is odd at a frequency as low as <200hz. This might have something to do with the internal resistance/capacitance of the NI 4461.
I appreciate the help.
04-17-2012 10:17 AM
I was able to look into your question some more and, unfortunately, the 4461 is not intended to deal with voltages in this manner. Especially with a third-party probe (which we can't validate ourselves), the attenuated signal from the probe has unknown characteristics - no known conditioning or output parameters. This being the case, we can't recommend using the 4461 for measuring voltages outside its rated capacity of +/-42.4V. The probes we that would be acceptable in terms of the attenuation conditioning, but cannot accept voltages at the 100Vpp level you are working with.
As mentioned, it's possible that one of the 4461's gain settings is causing your measured voltage to be lower than expected. However, we don’t recommend using the 4461 to measure signals ouside the +/- 42.4V range.
04-17-2012 10:58 PM
If possible can you do a small exercise with the same probe and card measuring different voltages starting from 1V with an equal interval step increment. I mean to say whether the card experiencing the attenuation at all voltages or not...
04-20-2012 12:41 PM
Thank you for the suggestion.
I have actually build a voltage divider out of two very large resistors which seems to work fine.
The circuit is simple, two resistors in series connected to ground in parallel to my main device. By attaching the voltage sense leads over a single resistor I can split the voltage in two (both resistors have the same rating). I calibrated this system and it seems to work.
Thank you for your suggestions.