USB-6009 analog input pegged to about 300mV

[zaphead]

I a USB 6009 daq module.  I am trying to read voltage from a temperature sensor (LM35). The sensor has three leads: one for power, one ground, and one for the output signal.  The output is in the range of -1 to 1 V.  I configure the 6009 for RSE input in that voltage range.  I power up the sensor with +5 volts and ground and measure the output signal using a multi-meter (one lead to ground, the other to the sensor output).  Measurements with the multi-meter confirm the sensor is working (output is about .2 V indicating about 20 degrees C temperature).

Next, I attach the output of the sensor to AI0 and attach the sensor ground to GND on the 6009.  As soon as I do this, the voltage output of the sensor jumps to about 0.3 V. I verify this voltage with the multi-meter.

I have tried multiple AI channels, two different daq modules, and multiple temperature sensors.  The behavior is always the same (voltage pegged to 0.3 V).

Any ideas what could be going on here?  Do I need to be concerned with impedance matching for this type of setup?  Thanks in advance.

[AnalogKid2DigitalMan]

zaphead:

See post #20 on this page, it may be of use, or do a search of LM35 on the boards:

http://forums.ni.com/ni/board/message?board.id=250&thread.id=16019&view=by_date_ascending&page=2

Hope it helps!

-Ak2DM

[zaphead]

Thank you for your response.  Yes, I've seen those posts.  Following those setups do not rectify the problem. I used setup illustrated in #1 of the attachment (kb.pdf).  Voltage pegged to 0.3V. I used the setup illustrated in #2 of the attachment.  Voltage pegged to -0.43V.  Note: the attachment illustrations indicate USB-6008.  I used the USB-6009.

Again, if I disconnect the sensor from the analog inputs (with either configuration), and measure Vout with a multimeter, the sensor output is correct. 

[zaphead · Accepted solution]

I now have this working.  I used RSE, connected Vout of the sensor to AI0+, connected AI0+ to ground through a 1.5k resistor.  Connected V+ of the sensor to external power supply (12V).  Connected ground on the sensor to ground on daq.  I have no idea why this works yet the other solutions posted online do not.  As long as it's working, I have no complaints.

[cassio]

Hello,
I would like to use my DAQ 600USB to measure the resistance wires. You have an example VI?
Thanks

[Dennis_Knutson]

Don't hijack a completely unrelated thread! You've done this twice.

Edit - the question was already asked and answered!

Message Edited by Dennis Knutson on 03-19-2010 08:47 AM