USB-6009 DAQ Question

[psgeorge]

I have a USB-6009 DAQ, Visual Studio 2005.net and Measurement Studio 8.1.  I'm able to read DC voltages in via analog input channels on the DAQ.  Could someone suggest the best method of reading AC voltages in?  I would like to read a single-sample rms value on demand.  I have a resister divider circuit to scale the AC voltage down.  What would be the best method of getting an rms value?

Thanks in advance for any thoughts and ideas!

[Kenn_N]

psgeorge,

Thanks for contacting National Instruments.  There will definitely be a need to externally scale the voltage to a level that the USB-6009 can read, which is in between -20V and 20V.

To determine the RMS voltage you need more than one point.  You can reference wikipedia on the actual formula, for RMS, but to calculate an accurate RMS you would need more than one sample.  However, if you know the peak voltage, and your signal is a true sine wave than you can use the common method of multiplying the peak by .707 or dividing by the square root of 2.  So all this depends on your signal and if it truly a sine wave and how accurate you want to be.  

Let me know if you have any further questions.

Regards,

[Tom_W_[DE]]

Hi All-

Just to add an additional suggestion- since you mention that you are using an external resistor divider network, you may need to consider the input impedance of the 6009 device (~144kOhm) and it's effect on the operation of your external circuitry.  It may be necessary to insert a buffer between your divider network and the input of the 6009.

Hopefully this helps-

[psgeorge]

I just got back from a small break over the holidays and saw the responses to my question.  Thanks to everyone for their comments and information.  This will help!