Usb 6008 + LM35 temperature sensor problem

[skogstrand]

Hi!

I have connected my LM35 Temperature sensor to my Usb 6008 DAQ on the ai0 port RSE. I have connected the inbuilt 5v powersupply and ground from USB 6008 to the sensor. My problem is that the output of the sensor reads a constant of ca 0,35 volt wich means around 35 degrees celsius on my chart. The output doesn't change when I heat up the sensor.I am wondering if the problem can be that since the USB 6008 is connected to my laptop and my laptop isn't grounded into housegrounding, my usb 6008 isn't grounded properly. Can this be the cause of the faulty measurement? I dont think this is a sensor problem since I have tried two sensors with the same result.

Thanks for any help.

[WM_John_Weeks]

The LM35 is about as simple a sensor as they come!

It looks to me like if you have +Vs connected to pin 31, GND to pin 32 and Vout to pin 2, it should just work.

Have you tried checking the output with a voltmeter? Maybe the problem is with your connections to the USB 6008 or with your program for reading it.

[skogstrand]

Thanks for your reply.

I have coupled it as you say. The Vout from the sensor reads a 0.35 v constant voltage in labview when connected with the USB 6008. Then I used a voltmeter to measure between ground on usb6008 and Vout on sensor while it was connected with usb 6008. There still was a constant voltage of 0.35v. Then I disconnected the sensor output from analog in on usb 6008 and measured the output from the sensor with the voltmeter and it worked perfectly. Why does the usb 6008 make my sensor output constant?

[WM_John_Weeks]

My best guess is that you've connected it to the wrong pin on the USB 6008. It might also be that the 6008 isn't working correctly.



Ah, ha! I just looked at the specs for the USB 6008- the analog inputs have an impedance of 144 kOhms. That's pretty similar to the resistances in your circuit, isn't it? You might need an amplifier between Vout and the 6008.

Added note:

I was thinking of another thread; sorry. That input impedance should be fine for an LM35 which has an output drive capability of 10 mA.

Message Edited by WM John Weeks on 09-21-2005 01:14 PM

[AnalogKid2DigitalMan]

Skogstrand:

I am currently baffled on the LM35 issue, could you post your schematic diagram including connections to the 6008?

Hi John:

Yes, after my resistance post, I thought I'd come back to the LM35 post. Found it could supply 10mA, so I agree that the 144K shouldn't be an issue.

[skogstrand]

Hi!

Here are the way I have coupled it in a paint file, and the VI program.

[WM_John_Weeks]

If you really connected it like your diagram, it should be correct. Make sure the the Vout wire isn't broken.

Hope the Digital Man can help with the VI.

[maxpower]

Try this - go into Measurement and Automation Explorer, select "My System", then "Devices and Interfaces", then select the device you're connected to (USB-6008).  On the right hand panel, select the "Test Panels" button, then under the "Analog Input" tab select the channel you are using, the mode, and the input configuration.  Run the test by pressing the start button.  By doing this, you can move the signal out pin from channel to channel and test to make sure that one of your AI channels isn't messed up.  Try moving the signal to another channel and perform the same test.  I've used several of these sensors over the years, and there have been times when I went through 4 to 5 before I got one that worked properly - especially if I wasn't careful when soldering the terminals.  I have two USB-6008 devices now that have a couple of channels that don't work properly, so it's possible that you may have a defective channel on the device.

good luck,
tsc

[skogstrand]

Thanks for all the replies.You are really trying to help.

I tested all inputs like you said in MAX, and they all read a constant voltage of 0,31v. But the sensor works when I disconnect it from the USB 6008, and measure with a regular voltmeter. Whats really weird is that I have tested my inputs on 6008 by connecting the outputs on the 6008 to them, and they work fine. What could be the difference between my voltmeter and the usb 6008 that makes only the voltmeter to work with the LM35?

[maxpower]

Try the attached vi out.  I'm curious if your problem has anything to do with the DAQ Assistant.

good luck,
tsc