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DAC0's output drop down after plugin


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#1 picknick

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Posted 29 August 2012 - 12:32 PM

Hello, I'm working with a U3-LV. I'm setting an output between DAC0 and GND of 3.3 V. For this porpuse I'm using Labview 2010 and running "U3 Multiple IO Example", a .vi downloaded from the LabJack-for-Labview page. When I connect the output to my circuit the voltage drops about 1 V (my circuit is just a led in serial with a variable load). I measure the current, 6 mA and the voltage between DAC0 and GND is around 2.2 V. When I vary the load the voltage and current vary too, but the power is fixed. So, I'm wondering... is the power of the LabJack just 12 mW? I can't found that on the specifications. Regards.

#2 LabJack Support

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Posted 29 August 2012 - 12:39 PM

The source impedance of the DAC outputs is about 50 ohms. That is the main cause of voltage drop under load:

http://labjack.com/s...uide/appendix-a

http://labjack.com/s...s-guide/2.7.1.1

http://labjack.com/s...t/app-notes/led

#3 picknick

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Posted 29 August 2012 - 01:01 PM

The source impedance of the DAC outputs is about 50 ohms. That is the main cause of voltage drop under load:

http://labjack.com/s...uide/appendix-a

http://labjack.com/s...s-guide/2.7.1.1

http://labjack.com/s...t/app-notes/led

Ok, thanks a lot! The fact is I'm testing a circuit (which is an optocoupler in fact), so a buffer should be fine, but... now I'm looking to the specs once again and I see that the maximum voltage output a DAC can deliver is 4.775 V at 2.5 mA so... Am I right? Is the power 12 mW?

#4 LabJack Support

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Posted 29 August 2012 - 01:52 PM

That is the max voltage you can expect if the load is drawing 2.5mA. That does not specify the max power output. If you connect a 50 ohm resistor from DAC0 to GND, and set DAC0 to 5.0 volts, you would expect about 50 mA, but the max short circuit current is 45 mA, so that is the most you can expect. If you get 45 mA through a 50 ohm resistor that is about 100 mW, so I would say a good estimate is that the "max power delivered to a load by a single DAC channel is 100 mW".

#5 LabJack Support

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Posted 29 August 2012 - 01:57 PM

I just did a test with my U3. I put a 47 ohm resistor from DAC0 to GND, set DAC0 to 5V, and got 1.7 volts across the resistor. That's about 60 mW.

#6 picknick

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Posted 29 August 2012 - 02:12 PM

I just did a test with my U3. I put a 47 ohm resistor from DAC0 to GND, set DAC0 to 5V, and got 1.7 volts across the resistor. That's about 60 mW.

Perfect! Thanks again for the answers and the fast response!


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