USB 6008 analog input

[JChec]

Hi,

I am driving a lm35 from a usb 6008. The lm35 requires less than 60uA and reads correctly if I check the voltage output to my multimeter. Eg. 0.24V which would be 24 deg. C. If I connect the output of the lm35 to my analog input on the 6008, the output jumps to 0.31V. Does anyone know why this is happening? Appreciate any response.

Thanks

[AnalogKid2DigitalMan]

JChec:

This may be useful, search results on lm35

http://forums.ni.com/ni/search?board_id=250&submitted=true&q=lm35

-AK2DM

[Brandon_B]

Hello JChec:

Thank you for using NI forums.  AK2DM has the right idea.  Especially with this particular post which goes in depth on several troubleshooting ideas.

Regards,

[JChec]

Thanks for the responses, didn't think to include the LM35 in the search, thought the problem was specifically the 6008. That particular post which BEZ refers to has 2 solutions written up. 1 solution using an external supply for the LM35.... which worked... the other being the resistor configuration using 120k resistors which didn't seem to work for me.

The problem seems to be with the grounding. Initially I thought I might just need more current than the 6008 can supply so I powered the LM35 using the 5V from another USB cable from the same computer which is powering the 6008, but the same problem occured as this 5V obviously has the same ground as the 6008.

Only when I used my benchtop power supply for the LM35 did it all work, until I connected the ground of the 6008 to the ground of the external PSU. So I'll have to go with isolated grounds

[JChec]

Thought I'd post my alternate solution to this problem in case this solution suits someone else... I didn't end up going with an external PSU.

I simply took a 5V - 5V, DC - DC converter to isolate the grounds between the LM35 and the 6008 and used differential inputs.

[sufre]

Hi  JChec, I have tried with you suggession to connect the +ve input of the LM35 and connect the ground from the external supply. It woks successfully. I check with DMM and Digital Thermometer which provide me similar reading. Thanks for you discovery... 

Regards

Sufre