Using USB6009 to read a signal from LM35

[Froes]

Hi,

I'm having a problem when i try to acquire a signal using the NI USB-6009.

I using it to read the voltage output from a LM35 (temperature sensor), but when i configure it to use RSE config, using both GND and AI0, i always get 0,32V, even with the lm35 without energy.

I tried to power the lm35 with the +5V from the 6009 and using an external supply, but there wasn't changes.

The voltage output should be around 0,26 witch is the temperature here now (26ºC).

I don't know what i'm doing wrong.

 Would you please help me?

--- Edited ---

I'm having the same problem using differential mode, using AI0 and AI1

[Rene24]

Hi Froes,

sorry i can't help you. But I have the same problem like you! Im really interessed to know why this happens. If i connect the sensor to a power supply and measure the voltage with a voltmeter the sensor give the right temperatur.

Hope someone has a solution.

René

[SnowMule]

The 600x DAQ's have a relatively low input impedance (150kΩ or so iirc), so the phenomenon you're seeing may be due to the output simply not being capable of driving that current.  Most DMM's have input impedances in the meg-ohm range, so they don't load the circuit nearly as much.

I have a test where I read the voltage from a photodiode to gauge distance/reflection of a material, and routing that wire directly into the input doesn't read any voltage, much like what you're seeing.  A simple op-amp voltage follower is in place to buffer that voltage and since the op-amp can pump the current for the input impedance, the distance/reflection sensor works great.

[AnalogKid2DigitalMan]

This has come up before, on the 1st page of searching LM35, there is a post with a solved solution (look for the green checkmark).

Hope this helps.

http://forums.ni.com/t5/forums/searchpage/tab/message/page/1?location=Board%3A250&q=LM35#message-lis...

-AK2DM