USB 6008 current limits

[VeeJay]

Hi,

I was just wondering if someone knows the current limitations on the NI-DAQ USB 6008 , for each of the channels (both analog I/o and digital I/0) went through the data sheet, but unable to determine each channels limit.

Thanks!

V

 

[johnsold]

The data sheet I have shows (on page 3) AO Output current drive <= 5 mA, Output impedance = 50 ohms.  Digital logic voltage levels are defined at 8.5 mA, so it would be reasonable to assume that no more than that should be drawn.

Lynn 

[VeeJay]

Thank you. But, looking on the DAQ card itself, I see that Analog has a max current of 50mA and Digital has a max current of 200mA.. What does that mean? And for these values that you have mentioned, are they for each chanel individually?

V

[johnsold]

The 200 mA is the maximum current which can be drawn from the 5 volt power output pin.  The 50 mA analog specification is the short circuit current.  That is not the amount of current which can be drawn while maintaining a programmed output voltage.  These specs are on the data sheet also.

The logic levels are per channel and the analog output drive of 5 mA is per channel.  Note also the output impedance may drop the voltage below the programmed amount if significant current is drawn.  For example if the output is programmed to be 4 V and 5 mA is drawn, the output voltage would be 3.75 V = 4 V - 0.005 A * 50 ohms.

Lynn 

Message Edited by johnsold on 01-13-2010 03:00 PM

[VeeJay]

I am so confused regarding the input current into Analog Inputs. It shows we  can use +/- 10V, but when i called NI, they said Analog Inputs can take only upto 0.243mA. There is also an input impedance of 144K ohms that I measured across the input and ground and it confirms the specifications on NI website. I have a question now. Please refer the file I have attached.

I have a sensor that is basically a potentiometer that varies from 30 ohm to 300 ohm. It works on 12V supply. Is the circuit in the picture correct if I want to limit the current into Analog input to within 0.243mA? R1 of 147K ohm is the input impedance of the DAQ AI. I have used R3 of 147 ohm (but can be varied to draw more current). R2 is the water level sensor with variable resistance. The input into AI is current.  So f I set this up for the DAQ, I hope I dont burn the card. Please help me out wit this.

Thanks!

V

[VeeJay]

oops forgot the attachment

[VeeJay]

oops forgot the attachment 🙂

[VeeJay]

I am really sorry for multiple posts and SOlution solved ticked... my comp is acting crazy and is hanging up... I dont know how to undo them.. 😞

[johnsold]

I thought you were talking about outputs.  For inputs you do not need to do anything special to limit the current provided that the voltage you connect to the input does not exceed the maximum rating.  The only time the input impedance becomes significant is when you are trying to measure a signal produced by a high impedance source.

If R3 is 147 kΩ and R2 is 200 Ω, then the voltage at the R2, R3 node is about 16 millivolts and, as you calculated, about 100 nA flows into the device.

If R3 is 147 Ω, not 147 kΩ, then the voltage is about 9 V and the input current will be 62 µA.  Either is much less than the limit.

So what is the question?

Lynn 

[VeeJay]

Thanks ! If I dont hv to worry about current for AI, that would be great. But, wht happens when your Analog Input is current? Then I do have to make sure that it is within the limit. Here, R1 is the input impedance that is part of the specifications. I can vary R3 to some constant value so that I get enough range of current for calibration. R2 is the water level sensor from 30ohm to 300 ohm. My concern is what value of R3 I should choose? Do I have to take the input impedance R1 into consideration to calculate R3 so that current into AI is within 0.243ma? Or, should I have R3 pretty high (disregarding R1) so that AI is within current limit?

I hope I am clear here.. I have attached pics for "if u dont consider input impedance" 

Also, If I hook it up on LabVIEW to view the exact measurement of current, will it take R1 into consideration and decrease the current further?

V